I've registered a hundred or so single word .ws domains in the last year from drops. Simply as keywords for SEO I think they have some value over reg fee. Even if I register a .ws at $10 and keep it 5 years I have $50 in it. Even if I only get $300 for it, that's a much greater rate than if I put the money in the bank. I do think a lot of them are easily worth $xxx if you are patient enough to wait for the right buyer, and not so greedy as to set asking prices at $xx,xxx.
It also depends on your cash flow and what you expect in return. Normally you won't get PPC income from .ws domains, so you can't count on making the renewal fees that way. The value is in finding a buyer who wants that keyword. The more of the gTLD's of that name that are really in use, the better chance you'll sell it in a reasonable time. Generally, if biz/info/us are available, pass on a .ws. If the at least 2-3 of the major TLD's are developed (not parked), then there probably is a buyer if you price it reasonably and the secondary gTLD's are highly priced. I sold a few last year in the $250-300 price range, with costs in the $10-50 range. I won't retire on it, but it is income and cash flow, as well as offers a little diversification.