Hmm... personally I'd say it depends. For shonen series like Naruto, Pokemon and the like, you can generally expect the English dub to be unimpressive or downright crappy.

But for something more seinen-oriented, e.g. Death Note or Code Geass, I personally found the English voice acting to be excellent.

That could be just from my limited viewpoint, though. Rule of thumb is look for the general opinion on the specific show's dub before you decide. If it does the original work justice, I usually go for it. It's just so much easier to understand and get into that way.