I agree. Here is Shimano's road lineup, according to their marketing hierarchy:

DURA-ACE®
The ultimate in road racing performance and dependability, born from leading-edge technology.
DURA-ACE TRACK®
Reflects the latest advances in Shimano materials and component engineering.
ULTEGRA®
Engineered to the same specifications as Dura-Ace for the best road and classic touring bikes.
105®
Our sport-racing group offering cost-performance value.
TIAGRA®
Real racing performance and enhanced feeling of control response for recreational road-sport bikes.
SORA®
An easy-riding road component group to make recreational cycling more fun.

My bike ranges between 105 and Ultegra (no Tiagra or Sora) and I have had no complaints in several hundred miles. Some folks say the bike frame is the foundation, and therefore most important consideration. However, I think as long as a frame is decent, your drivetrain type stuff (particularly shifters and derailleurs, and MOST particularly the rear derailleur) is going to have the largest influence on your bike's performance and your satisfaction/enjoyment.

If you are looking for a decent bike on a budget, I'd possibly look at one of the more generic name brands, like what you would see at Sun n Ski. Or look at respectable higher-production bikes like Trek. You may get same general level of frame technology, and save a few hundred bucks as compared with similarly spec'd other bikes. For example, a Trek with a 105/ultegra group might be a few hundred less than another 105/ultegra-equipped bike.

Ebay is great. I even sold my car on eBay. But I'd be wary of buying a brand that you cannot find or test-ride at a local shop.