You'll need to let us know a couple more relevant things to answer your question properly. It's not just the price that's a major factor in a bike purchase...

• Is this gonna be your very first bike purchase, or have you owned several?
• What sort of budget have you set?
• How long have you been riding or are you just starting out?
• Are you competent to set up the mechanicals of the bike?
• What type of usage and distances are you looking at?

These sorts of things can possibly influence your decision whether to buy on-line or not. The more experienced you are with bikes and cycling, then the more likely I'd be to recommend buying on-line. I's also dependent to a certain degree on what type of bike you're thinking about; a $350 hybrid or a $3500 carbon racer. El-cheapo's okay online, but upper end? Mmmm...

However, if you're a novice, then I'd say a visit to your LBS is the best (only?) plan.

Cheers.