Anchros ROM or "True" Android? Simple Question?

OK. New here. I took the plunge after a lot of research (I almost got a Nook Color) and ordered a Archos 70. I will probably see it on Tuesday.

So it seems like a lot of people are after a different "more pure?" Android ROM. Is there that much to gain from going to Voltaire or another non-Archos ROM? What are the advantages to the "upgrading" the ROM? Are there any dis-advantages?

Thanks in advance.

- W