Well, it's fair to say they don't completely destroy the phone. They have been making things less pleasant at least since the 6230, though. The Cingular version of that phone crashed on occasion. Once I had mine unbranded, it never crashed.
The E62 is crippled in the hardware department. The software isn't actually that bad, unless you want to use Java apps, since you'll run into the annoyingly never fixed bug of Java network access doom. Well, other than the occasional random reboot, although that's pretty rare, and the much more irritating misfeature of going into speakerphone when answering a call and then locking up after the call is over that happens to me weekly.
I haven't seen an unbranded 6682, so I don't know whether it would crash less without the Cingular software or not.