I've had my Zumo 550 for a few weeks now and have been keeping some notes on it and thought I'd pass them along to other potential buyers.
Let me start off by saying overall I love this unit. If I were shopping for one for just my car this wouldn't have been my 1st choice. But for water-proof units on a motorcycle, I think it's hands down the best option out there right now. Also, in the past I've used USB GPS adapters tied to laptops mostly which have advantages in flexibility, display size, features, up-to-date maps, but significant disadvantages in areas like portability, and convenience. In the end I'm happy with the purchase.
Pros
It's great that it comes with car and motorcycle mounts; RAM mount is awesome for the bike, although I didn't use the handlebar mount, I anchored their RAM mount to an existing RAM ball on my steering stem nut)
Plays MP3s while viewing other screens like the map, settings, etc. (not all gps's can do that; some require you to look at the mp3 player screen the whole time) It pauses the audio whenever giving direction prompts, or anytime you're on your bluetooth cell phone. As soon as it's done talking, or when you hang up the phone, it picks back up instantly.
Auto-adjusts to motorcycle/car mounts for keyboard preferences (i.e. use bigger glove-friendly buttons on the bike only)
Good sound quality from the MP3 player
Fast satalite aquisition time and rarely loses GPS signal. I've never lost signal going under a bridge yet. This happened to me sometimes with my laptop unit. It even works inside my house within several feet of the windows.
Does a good job of text-to-speech overall and is understandable (only mispronounced one street for me so far and I'm not even sure how to say that street. It does have trouble with abbreviations sometimes and will say "approaching H-W-Y three twenty one" instead of "approaching highway three twenty one, but no biggie)
No hard-drive or spinning media to die from motorcycle vibration.
Expandable storage via SD cards is awesome. I have a 1GB for audiobooks and a 2GB for tunes. I use the tunes on bike rides in shuffle mode usually. Then I use the 1GB for audibooks when in the car on business trips. Helps that these are very cheap now-adays also...
The ability to load custom POIs is nice. You can use the Garmin POI loader with TONS of free point-of-interest lists from various GPS sites. In ~30 minutes I had located a slew of websites with lists and loaded all of the Starbucks/Targets/Walmarts/Lowes in my unit
The ability to pre-load a motorcycle ride from your PC is great. In the MapSource software, I like how you can save all your routes and "tracks" into one file and view them simultaneously or individually. The ability to pre-plan long rides like that is a HUGE boost to the enjoyability of your ride. Also, if you want to take a 3hr ride this weekend, just pull up MapSource at your house, and start scanning the map for potential routes and click on them as you go. It adds up the distance and time until you get your 3hr ride. Send it to the GPS and it'll tell you where/when to turn all along the way, allowing you to focus on the ride and your safety.
Bluetooth works great with my Treo 650 and Plantronics headset. I don't often use my headset with it though. But it's nice when in the car or on the bike to get a loud audible alert over the radio/headphones that a call is coming in with caller-id, etc. Then I can pull over on the bike if it's important, or return the call at my next stop.
Water-proof
Cons
GPS
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I've had it try to do some funky things to get me onto my route. Like while I'm on my route, heading in the right direction, it would try to get me to do a u-turn to get onto the route. I keep driving and it figures it out within 10-15 seconds. Or in one instance I was on a route, pulled off for gas, and instead of it having me get right back on the highway where I got off, it tried to route me through a few back streets to a different exit. The only other routing issue I've had is it suddenly thinking I was on the northbound side of I-85 when I was actually on the south-bound side and traveling south. I've had other products leap me 10-15meters one way or the other before and tell me I was off-route due to a GPS signal glitch, but was surprised to see this one tell me to get off the highway at an exit and get back on in the right direction. It thought I was driving the wrong-way down the highway!
All that being said, I've had similar experiences with other products, and with any GPS you obviously have to still use some common sense when it tells you to go somewhere or do something. It's an aid, not an auto-pilot, after all. All in all it does a good job and recalculates routes quickly.
MP3 Player
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First off, you have to understand that this unit is MEANT to be a GPS 1st and an MP3 player 2nd (or 3rd maybe after XM radio). But for those of you looking to use it as a replacement to an iPod or other portable player, just know it's not going to be as feature rich.
It doesn't support playlists. This isn't an issue for when you're out riding the motorcycle since I usually just put all my music on shuffle and cruise. However, if you want to play one CD, or if you want to play all the tracks from a collection (or audiobook) then it's a pain to not be able to seperate them out and play them in a certain order. I originally thought this wouldn't be an issue and I'd over-come it by just ordering my files via their filename. Then I could tell it to find all files in an album and hit play all. Well, it doesn't play the files in any discernable order that I've found yet. It doesn't do it by filename, date, modified date, etc. So I've yet to find a way to play files in a specific order 100% reliably. Sometimes it'll play a folder in the order I have it listed "track1.mp3, track2.mp3, track3.mp3, etc" as if it sorts them by name using the #s to get the ascending order. For other folders it will appear semi-random like "track1.mp3, track61.mp3, track53.mp3, track2.mp3, track62.mp3, track54.mp3, etc" where they sort-of increment but not in a linear order.
Also, there are no EQ or bass/treble settings. It's not a huge issue if you're piping it to the radio on your car as you can adjust to your liking there. But for the bike, if you're using earbuds or a comms system, you have no way to tweak it.
Lastly, there are no bookmarking abilities. So if you're 45 minutes into a 4hour audiobook, you can't bookmark it and restart your playback from there later except for when you power the unit off. It does power back on and resume play from the previos point. But if you change songs, etc, your place is lost.
Software
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More difficult to build/develop routes than in Microsoft Streets or Delorme Street Atlas (enter house # in one box, select a street from the list, select a city from the list, state from the list, etc; can't just type in "123 my st, my city, sc" as a source/destination) (Can't easily add a "via" point to an existing route; you have to search for the location and then add it as a waypoint. Can't just click the map on the location)
Maps are not as up-to-date as Street Atlas (found several local roads that aren't in it yet). The maps seem consistant with the data you see on Google Maps or Microsoft Streets though.
Avoidances aren't 100% avoided on the unit. If you tell it to avoid highways, it does so as long as it doesn't have to add considerable time to the trip. However, for motorcycle rides, you WANT extended times/distances on back roads in most cases. This can be overcome if you pre-plan the routes in the MapSource software on your PC though. You can tell it to avoid a specifid road or area. You can also force it to route you down specific roads that you like. This is what I do on pre-planned motorcycle rides. In the car, I don't care, I just want to get there ASAP and it does that well.
Anyway, those are the things that have jumped out at me so far...