Guitar Center is good for:
1) last-minute accessories and small things like picks, strings, cables, guitar straps, etc.
2) Testing out instruments and amps to get a general sense of the differences between certain models or brands. But only on weekday or weekend mornings when it's not crowded and noisy.
3) SOMETIMES, there is the occasional sales rep who is knowledgeable and not trying to pawn off whatever they have in the store so they can get their commission.
If you want to buy online, here's a couple of decent recommended ones:
RECORDING GEAR
www.mercenary.com - in MA. carries mostly high-end mics, pres, converters, and some used stuff; some budget stuff.
www.atlasproaudio.com - in FL. similar to Mercenary Audio, has some used gear
www.humbuckermusic.com - small dealer that provides good value-for-money gear
www.bayviewproaudio.com - in WA. Deals in everything from high-end to budget stuff
www.altomusic.com - in FL(?). another small dealer that provides mostly high-end stuff
www.8thstreet.com - in NJ. An online superstore that provides a wide range of recording gear from budget to high-end (and instruments, see below).
www.zzounds.com/a--2676837 - in NJ. Another online superstore similar to 8thstreet.com
www.bhphoto.com - in NY. Another dealer in higher end recording gear
www.sweetwater.com - in NJ (?). Huge selection, prices not always the best though; you are assigned a 'salesperson' who you buy from; like Guitar Center, people either have great experiences with them or horrible experiences
www.musiciansfriend.com - owned by Guitar Center. Better selection than the stores, better prices (but not the lowest prices online). Some buyers have had gripes about bad product being shipped, and returns policy that can be difficult.
MUSICAL INSTRUMENTS, AMPS, AND ACCESSORIES
www.8thstreet.com
www.zzounds.com/a--2676837
www.sweetwater.com
www.musiciansfriend.com (same company as Guitar Center; lots of mixed experiences from users)
USED INSTRUMENTS AND RECORDING GEAR
www.mercenary.com
www.atlasproaudio.com
www.craigslist.org - if you're close to a major metro area (NY, SF, LA, etc.) this could be a good place to look for locals selling their used gear
And last but not least, you probably know there's always Ebay.
Hope this helps