How important is the translation to your reading, when you're reading an ancient work?
For example, I've had people tell me that the Neville Coghill translation of The Canterbury Tales isn't a good one and that I should find a different translation. I have no idea if this is just scholarly snobbery though (ie the "Newer is Better" way of thinking) or if there is something to it. I keep the translation for a different reason (besides the fact that I like it): Neville Coghill was a member of the Inklings, the group that Tolkien belonged to.

Right now I'm reading Beowulf (along with all of the other books I have on the go), and I'm really finding that the translation makes a big difference.

However, there really is a difference. Seamus Heaney has tried, more or less, to hold to the alliteration of the original Beowulf, but has, as far as I can see, chosen to not reproduce some of the rest of the structure of the poem. There is also, in my opinion, a more casual feel to his translation. Most of the book is the poem, which is probably a plus for the causal reader. I know I liked it, being able to jump right into the poem, when I first got the book back in 2004.

I haven't made it as far as the commentary yet, being just over 1200 lines into the poem, but on a quick skim, I noted that it includes a detailed history of the manuscript history.
What I'd like to find for Beowulf is a translation that does what Tolkien did for the poem Sir Gawain And The Green Knight: His translation includes both the alliteration and the poem structure. Of the translations I've read, this has become my favorite (and not simply because it is one of the works of J.R.R. Tolkien this time).
So, what makes a good translation for you?