I used it for a while.
The filtering system is not the greatest. I wanted to set up a complex set of filters to handle some RSS feeds, and it sucked very badly; it applied some of the filters but not others, and it partially applied other filters. Filters were also order specific... look, the filtering system just sucked balls. Outlook actually has the filters pretty downpat.
That being said, actually handling emails - works great. I've never had a problem with it. Between Thunderbird and Outlook, in terms of pure functionality neither one has any distinct advantage over the other. They both allow you to check your mail. They both allow you to read your mail. They both allow you to send mail, and have address book functionality. Thunderbird does have one SIGNIFICANT, and I really mean significant, advantage over Outlook 2007 though.
If your PST file gets exceedingly large in outlook, e.g. you have over 1 gb of mail, and outlook crashes for whatever reason, you can lose those emails because Outlook needs to close that PST file carefully. It's essentially an open file handle for the entire time you're using Outlook. This basically means if Outlook crashes, you not only stand a good chance of losing all your emails not stored on the server, but it also takes forever to check the file (up to 15 minutes), in which time it will be hammering your CPU and hard drive, aka bringing your entire windows system to a grinding halt. Sure, you can just delete your old emails or archive them... but I have a lot of emails from when I first started as a new hire at work that I can't delete, little tricks of the trade that aren't written down anywhere, etc. so archiving them would cause me much pain...
EDIT: at work I use Outlook, because it integrates with my phone and Exchange. At home however I use something called PopPeeper, which is an extremely light weight client for checking emails. I think it's a matter of kB instead of MB in terms of on-disk space. It's absolutely terrible for features - it has check mail, delete mail, and if you choose that setting, send mail, but there's no viruses out there that can attack it because it's so backwards, and for bulk email accounts and processing emails quickly it's great (starts up in less than a second, allows me to save all settings to a file, does the job and NOTHING MORE). The memory hit is extremely low so I prefer to keep this open compared to Outlook 2007.