I actually blogged
about this recently, after Llodra won a few matches and re-ignited the s&v debate at the pro level.
I think it can be an extremely effective tactic at the 3.5 level because, as Drop Shot noted, many players aren't used to facing it and you can win a LOT of easy points. The problem as I see it though is the physical toll it takes. When you serve and volley you make a run to the net on every first serve - whether your serve is in, into the net, or into the back fence. It can be exhausting for 3.5 players because, frankly, they're going to miss a lot of first serves and therefore do a lot of extra running. And if they aren't missing a lot of first serves then their first serve probably isn't forceful and they'll get consistently passed by the better 3.5 players.
Still, as I noted in my blog post, I think it can be a terrific strategy at the club level - especially when applied against the right opponent.