I upgraded from an old Tiagra rear derailleur to a new 105, and it made a decent difference. I've had the 105 for a couple years of heavy, all-season commuting, and it's holding up fine. I got it on sale for about $25, so it was definitely worth it. I don't think it would be worth the difference for my non-indexed front derailleur where precision is less important.

For commuting, I like my dura-ace bar end shifters (the only kind?). I would not want the 105's that work on the brake levers because sometimes I have to work them with mittens. Also, if I have shifter problems, I want to be able to replace only my shifters. If I have brake problems, I want to be able to replace only my breaks.