[beambot]

You'd have to be really careful about that. arXiv.org isn't peer reviewed, so there's a lot of crackpot science on there. For some of the best examples, search for P!=NP proofs.

(It also depends on discipline. A lot of respectable math and physics preprints first appear on arXiv.org and can be exceptionally solid.)

[jmmcd]

Moreover, it's skewed by which disciplines tend to publish in arXiv, as opposed to more traditional venues, ie conferences and journals. Physics has always been the main mover on arXiv, computer science surprisingly (?) rare.