I haven't really seen much dubbed anime and the animes I did watch in english I haven't rewatched in japanese simply because I don't really like rewatching anime that much.
So I don't really have much ground to compare, but I can atleast say if I enjoyed the animes of which I did watch the dubbed versions:
The melancholy of haruhi suzumiya:
One of the first 'real' animes I've watched, it was one of the animes that got me into the entire anime scene.
When I started out I wanted english versions 'no matter what'... yeah I was one of those pathetic guys

Anyway regarding the Haruhi dub I can really only say this:
Haruhi's english voice was really cheerfully and pushy and it fitted her character perfectly but..
Kyon's voice was what I liked the most! His intonation and the way he talked just fitted his sarcastic character so perfectly it's part of the reason why I like this anime soooo much.
I think its good that I watched the english dub for Haruhi (I downloaded the dual-audio version anyway) because back in those days I didn't really know any Japanese anyway
so I think it would be hard for me to really 'enjoy' the sarcastic nature of the Japanese Kyon, but english I knew very well so I could easily enjoy that.
As stated before I haven't watched the Japanese version so I don't know how that works out but I can sure say that atleast (in my humble opinion) the English dub is atleas worth watching for people
that are new to anime and/or aren't really into the whole 'japanese language' thing (even though I now admit it's a big part of what I love about anime)
Gundam Wing:
I said Haruhi was the first 'real' anime I've seen.. but I watched Gundam Wing when I was ~16 or something years old and I watched it on TV.
Naturally it was the english version that they aired (with dutch subtitles) But I really loved the anime for its 'deeper meaning' and all its psychological talk.
Overall I enjoyed the anime a lot, and I think the voices fitted the 'darker' and emo-ish outlook that the anime portrayed quite well.
Naturally I haven't seen the original japanese version so I don't know about those voices but I can atleast say I enjoyed the english voice cast a lot and
I can't really remember a point in time where I felt the voices were out of place or something like that.
There are some other animes I did watch in english but those are mainly 'main stream' animes that made it big in the western world anyway (think DBZ, Yu-Gi-Oh, Digimon, Pokémon etc).
So there's not really much sense in giving my opinion about those dubs because EVERYBODY knows how those dubs are like and a lot of you people here have probably seen those sort of series
yourself on tv while you were young.