Pixie bike FTW :thumbup:

Biggest thing for me when I buy a bike is fit. If it's too small or too big, you won't be comfortable riding it. For a mountain bike, when you stand on the ground, the top bar should be ~1-3" from your crotch. The seat should be able to adjust ~ 6-12" above your butt when standing.

Also, is this bike for commuting only? If it is, you might consider road bikes instead of mountain bikes, which will be easier to ride fast (I can do about 26 mph on level ground with full effort, but I can do this on a road bike with about half effort).

As far as pricing goes, the old adage you get what you pay for holds true here. As price increases, component groups become more robust and lighter. However, unless your commute involves stairs, your average Huffy bike will do just fine for commuting. Take in a trail or race it, and you're likely to break something. Check craigslist for good deals on used bikes.