I can't speak for skitucs but I just put Polk Audio db651s's (6 1/2") in my front and rear doors. They do sound a lot more clear than the stock speakers. They fit nicely and don't block the windows from rolling down.
I'm pretty happy with them. The only trouble I had was with the brackets that Crutchfield includes for the installation. I don't know if I was doing it wrong but most of the holes didn't line up. I ended up having to drill new holes to mount the things. The speakers aren't to blame for this, though. The brackets are meant to be somewhat of a universal fit, I assume, so it's fair to expect some modification to be required. The whole install took about three hours, but I also applied some of my spare Dynamat Extreme to parts of the door and trim.
I still haven't adjusted my receiver to optimize the EQ so hopefully I'll be able to squeeze out a little more quality from these things. I installed my subwoofer on the same day so there's quite a bit of adjusting left to do. At $150 for a complete set, with installation gear (those harness adapters are a life saver), I'm happy.
Update: I had a chance to mess around with the EQ and other sound processing features on my head unit (Kenwood KIV-700) and it really brought these speakers to life. I am 100% satisfied with my purchase. I finally got a nice balance between the speakers and my subwoofer. The Polk Audio speakers are very clear and vivid. I may do some very minor tweaking with the sub tomorrow (just to add a tad more thickness to the overall sound) but I think the speakers are just right.
My receiver has a load of different settings to adjust so that helps a lot in getting good sound. There are so many parameters that can be tweaked that it makes it an almost intimidating process. I think I finally got a grip on it, though.
There's no doubt that spending more money on speakers would yield even better results but dang, $150 for four of these is a steal! They sound very nice and they fit really well in the car. No problems to report on my end!