I'm going through the same thing with the Blackjack right now. The 3G speed is wonderful. WM5 Smartphone is awkward and buggy. I wouldn't quite go so far as to say "unusable", but it is incredibly frustrating. Samsung also made some really bad design decisions. Give me Symbian or Blackberry any day over WM5.
My Cingular SIM will probably go back in my E70-2 once I get sufficiently frustrated with the Blackjack. If I need UMTS for tethering, the Sierra Aircard 860 I just bought will do the trick.