This is a simple steps to fix it (or so for me) :
1) Go to recovery and Wipe ALL
2) Flash CM6 RC1 AND Google Apps VERSION 3 ONLY!!
3) Reboot out of recovery and let the CM6 load all the way. (Note: it will reboot itself, from some problem)
4) Boot into recovery (Take out battery and put it back in if bootlooping)
5) Wipe data/factory (only)
6) Reboot.
7) Tada! Should not boot-loop any longer.
8) Run FIX PERMISSIONS in ROM MANAGER
9) Reboot.
Sorry if I misspelled some things as I'm writing. This on my mytouch with cm6
Okay, so you guys might have experienced either of these, because I certainly did. But remember, I might have started out differently.
Problems I had:
When upgrading: bootlooping kept happening after a single boot. Lockscreen was stretched. Marketplace didn't work. Nothing synched. Didn't allow to add google account later. Only corporate. None of the google apps work, did i say that already?
After a fresh entire wipe installation: tapping the android setup didn't do **** until a lag later. Keep tapping that android to get utterly working!

eventually it'll stop lagging and respond.
Kept bootloping after A few seconds of sucessful load. And had a few Google app problems.
My info:
During upgrade I had the final 5.0.8.
I'm running it with a mytouch 3g (headjack, 1.2) . I'm using amra recovery 1.7.0, flash it please

. Another thinng to remember is make sure you have the extra stuff like danger spl ONLY IF YOUR DEVICE REQUIRES IT. Check the official thread for that.
Before I had cm6: I had 5.0.8, ive been using it for a while since it first came out. So as you follow along, keep in mind that I upgraded from a 5.0.8
What I did was:
*1) boot in recovery mode.
2) nandroid backup
3)flashed the cm6 and gapps VERSION 3 (currently the latest)
4) rebooted, loaded fine. Everything was going okay even the market place. Few things didnt work was that once I rebooted it went into a bootloop.
DO NOT DO THESE STEPS. PLEASE READ IT ANYTHING BEFORE YOU DO ANYTHING!
So I started fresh..
**1) booted into recovery ( I took out the batteries to get out of the dumb bootloop)
2) wiped every single thing on the list.
3) flashed cm6 and gapps version 3.
4) rebooted and when I got into locksxreen or setup it rebooted itself .
5) it did the same effect as 4.
Possible fix?
6) booted into recovery.
7) wiped data (only, usually first one on the list).
8)rebooted.
9) worked without bootlooping!
Other things I did...
10) went into rom manager and used fix permissions.
11)rebooted to recovery.
12) nandroid, so you wont have to do this all again because I don't have an exact starting point that fixed this for me.
13) went to cyanogenmod settings and actives ALL the performmamce tweak for the hell of it. Even 64mb
14) rebooted, sweet and smooth. Let the first time boot tload up before ypi start tapping like **** everywhere
Everything in cm6 works for me. Evven the performance tweaks and it is running smoother than ever!
If you have questions please. Ask, I hopes this helps someone, I know its kinda confusing, but I guess I would suggest starting from step **1.
Man my wrist is aching mytouchhas such a small screen for typing this.
UPDATE: after I did all this so far the only problems left is..
1)click on the weather icon, you'll notice when you drag accross the timeline it wont change the data above.
More to come later