Perfectly fine on my protege5.
I just got the identical one in for the 3 and I installed it.
I don't listen to the radio much but when I do it doesn't seem to be an issue. If anything I'd say 5-10% reduction over stock?
If you live in the boondocks then a really good antenna may be more critical. In that case, stock is your best bet.
Shorter may not be better but I'm not going back to the stock antenna. I think the CX7 antenna is too long to my eye. Proportion is important and I feel the 3.5 is a nice look. There are even 1" ones that seem too short and are probably not much better than no antenna.
On the other hand, you can always get some FM antenna wire and string it up underneath the headliner and have the roof antenna just for show.