I was extremely skeptical of LinkedIn but unlike Stowe and Jeff have found it to be pretty valuable. First for recruitment - the quality of the candidates I have seen through advertising on LinkedIn are fantastic - and I like the fact that some of them come recommended by people I trust. Second, it has proven to be immensely valuable in connecting with old colleagues and keeping my current network humming along.
Sure, the spam is annoying. I view that is the price we all pay for participating in relatively open communities. So, to answer the question -" has it been useful" - absolutely.
But I also agree with Jeff that if you can't see direct benefits, you probably should opt out. Doesn't that apply to anything and everything?