Well I've just fitted the Pioneer upgrade speakers (TS-Q131C), £70 from Car Audio Direct. The woofers sound great, but the tweeters are worse than the originals. So I'm running a Frankenstein setup, you could say!
The woofers just slot straight in, but the tweeters (if you choose to fit them) don't fit quite properly, they're too deep and so the cover sits a little proud of the dash. One thing to watch out for is if the car actually has tweeters and/or the wiring for them in place, I know some don't.
The bass is very tight with the Pioneers, I generally listen to rock and some electronic, testing this evening after fitting was done with the Foo Fighters. I can now hear the difference between the bass drum, toms and the bass guitar - this was previously just "mush" with the old woofers.
I think general consensus on here though is if you have the regular Update List headunit, upgrading the speakers is fruitless as there just isn't enough power to drive them properly. I know with hifi you can get 30 watt amplifiers that sound great (my dad's 1980s Arcam A60 springs to mind) but generally that doesn't translate well to car audio.
You may therefore get more mileage from upgrading the headunit.
Cheers
Joe