Buy used, my only bike so far was bought that way. Saved upwards of $400 dollars and got a pretty good upper entry/low mid ranged road bike(alloy Cannondale Synapse Tiagra) for 800. The alternative would most likely be me buying a new lesser model for the same 800 and craving a new one sooner. Haven't had any problems with it that weren't caused by me own stupidity, the shop sold it with the same perks that come with a new purchase, and its all the bike I need at the moment. A little over a year and close to 4,000 miles on it.
The thing is, many of the components that will get used the most, you'll want to personalize yourself anyways. Here are some things I've felt the inclination to change:
Wheels - The wheels didn't feel 100% new, but it didn't really matter. A little truing and rim tape did the trick. I upgraded when my bank account allowed for it. Even new they were a cheap $100 set and the upgrade 8 months later was fantastic.
Tires- Again, not 100%, but good enough. You'll want to pick tires at a later time on preferences anyways.
Bar Tape- Depending on the usage, you may want to get new bar tape. The stuff is cheap and makes your bike look a little cleaner.
Saddle- Haven't changed it yet, but it is a little scratched up/beat up. I looked it up this week and the stock saddle is only $40 new....Know wonder its so uncomfortable on longer rides. Just like the others, I'd most likely look to replace if it were a new purchase as well.
A grand total of +- 700 bucks for all those upgrades to make a used bike function as well as a higher model in the $2000 range. Buy used and upgrade/customize with the savings.