The Subs Vs. Dubs debate is rather predictable. I've seen it pop up counteless times on countless forums. Each time I would carefully articualte my response, only for the thread to fade into the background (or even get deleted), and another to pop up in its place. I eventually grew tired of rehashing the same points, so I created this; my own Copy-N-Paste response that works perfetly in any such thread with little to no adjustments.
I generally do prefer dubs. I'll happily tolerate a mediocre dub (i.e. Elfen Lied) simply for ease of viewing. I have no problem with watching something subbed. The first fansub I ever saw was a subbed VHS of Gundam Wing I found in Chinatown (New York) when I was 11, and I've been watching foreign films (with subtitles; Live Action films get a special "Death to Dubs" clause in my book) since I could read. However I will generally try the Dub first, and then decide which to watch based on what I hear. There are plenty of cases where I've originally tried the Dub and simply stuck with it (Evangelion, Ghost in the Shell) because I had no problem with the English voice actors. Likewise there are plenty of times where I've tried watching something in English, only to switch to subs (Clannad and Saikano come to mind).
One constant sub-only argument is that English dubs tend to be Macekred. I remember the 90's, and yes, it did happen. But that just doesn't happen all that much anymore except in certain instances (such as shows that are syndicated in the U.S. with dubs meant for younger audiences). Macekring has become the exception rather than the rule. These days, changes in dialogue are rather insignificant, and can sometimes even improve the original work (Balalaika's confrontation with Hansel in episode 15 of Black Lagoon is a prime example.) The most extreme changes tend to come from adapting jokes, such as Yukari of Azumanga Daioh going from an English teacher to a "Language" teacher in the dub. Sometimes these changes don't work, but Azumanga Daioh pulled it off pretty well.
Sometimes the improvements aren't even that obvious. In the Japanese version of Kino's Journey, Hermes (a talking motorcycle, for those unfamiliar with the series) is given a pretty standard voiceover. It's good, but it sounds just like any other voice role. Contrast this to the English dub, where ADV cast Cythia Martinez, and altered her already throaty voice with a metallic reverberation effect. The result is a voice that the viewer is much more willing to believe is coming out of a talking motorcycle.
Then of course there are some English dubs that, regardless of the quality, are just more believable because of the setting (Gunsmith Cats, Baccano!, Chrono Crusade, Hellsing Ultimate). Watching characters who are supposed to be in your own country speak in a completely different language is a little jarring, to say the least. This also applies to titles where, while they're not necessarily set in an English speaking country, it's implied that the intended language is English (Several of the jokes in Black Lagoon as well as the nationalities of the majority of the characters point to English being the intended language, and the fact that Ross Syllabus is from Chicago coupled with the all-English signs in Armitage III suggest that they mostly speak English on Mars).
Ghibli's movies earn a special mention, as the current Buena-Vista/Disney dubs (and any changes made) have to be approved by Ghibli, and they are not allowed to cut or alter a single frame. On top of that, I've yet to find a Disney dub of a Ghibli film that I didn't like (save for Princess Mononke, but I blame Billy Bob Thornton for that one), and for most of them I've found the English versions to be just plain better (My first reaction to hearing the Nausicaa dub was "Holy $#!* it's Patrick Stewart", who could win an Oscar for reading the phone book aloud. And I thought that Tina Fey and Liam Neeson's performances in Ponyo absolutely made the film).
Another reason I tend to prefer dubs it the fact that many of my favorite series are very art-oriented. When watching something such as Millenium Actress or Gankutsuou, where the visuals are one of, if not the main attraction, dubs allow the viewers to immerse themselves into the visual side of the work without the beautiful scenery being marred by lines of text. Voices of a Distant Star earns a special mention here; the gorgeous scenery (especially in the sequence on Argatha) is half the reason for watching the film. Unfortunately, the characters talk rather quickly, leaving irritating walls of text on the screen. While the English dub isn't wonderful, it's very tolerable and allows the viewer to observe Shinkai's visuals without any obstruction. Of course, this principle means that I find it even easier to watch something subbed if the art is only "OK", such as When They Cry (I loved the story and the suspense that came from trying to figure out the various mysteries within it as the show progressed, but the animation's budget shows and it leaves a little to be desired).
Of course, there are a few of my favorite titles that I dread the thought of being dubbed simply because the original cast was perfect (Library War), or because I've already bought the sub-only DVD (The Skull Man) and don't want to shell out money for a second copy.