I ride Tiagra. It shifts fine, I need to adjust it but then again so does every groupset. I've test ridden 105 from the LBS, no noticable difference yet.
Tiagra works just as well as any other groupset, especially if you're not racing. Just make sure it's well taken care of and well adjusted, and it'll do fine.
In retrospect, Tiagra does have one major flaw... how the hell do you pronounce it???