I'm still a noob here (and effectively to cycling in general) but I'll throw in my $.02.

I was looking at MTB's (for mostly commuting). I went in looking at a Rockhopper, but it just felt wrong. Didn't like it at all. They put me on a Fisher 29er (Cobia), but it was out of my budget (though it felt much better). Went back home to research and half decided on a Hybrid, it seemed like I could get a lot more bike for the money (after realistically thinking of what I actually needed). With a ~$500 budget I wanted to try a Raleigh Passage 6.5, but no one had one in stock. Didn't like the models below it. I ended up with a Trek FX 7.3. It could use a more bike path friendly tire (35c instead of 32c slicks it came with), but it doesn't do too bad considering. The 7.2 was cheaper and had the bigger/treaded tire I want, but the 7.3 (just over $500 full price) seemed like money well spent otherwise.

http://www2.trekbikes.com/bikes/bike...d=1327010&f=26

I haven't been on any long rides yet, but it seems to be much more friendly for street use than any of the MTBs I rode (at shops and friend's). I decided to go more for my needs (commuting on road and bike paths) than my wants (potential off-roading).

My main advice? Go ride them! One bike might come highly recommended but just feel all wrong to you. Ride a mountain bike, ride a hybrid...see which feels better to you. It's difficult to get a great feel on a short test ride, but I bet you'll be surprised just how different they can be both between types as well as different brands of the same type.

I'm sure some may disagree w/ my choice, but it seems great so far. There are better bikes for my needs, I'm certain, but I didn't find them w/in my price range.