I'd say bad. When you tell someone about your site, you have to say "your dash site dot com", which is not only longer but also harder to remember. (This is similar to the GNU/Linux debate in some respects.)
If you buy "your-site.com" to protect "yoursite.com", which you already own, then it's worth it.
Good point...how would you quickly tell someone to visit www.slash-dot.com? Or www.50-yard-dash.com. For that last one, I'd hate to have to always say "50 [as in the number five zero] dash [the symbol] yard dash [the symbol] dash [the word] dot com"
i find the - more pleasing but after having this conversation w/ many many people over the years, i am obviously in the minority. most people i know would assume that your non-dash name was taken and you were desperate to get some name in that variety. take from that what you will.
The - can help eliminate some confusion www.pen-island.com would be more clear for a ballpoint pen website than www.penisland.com, which has another interpretation. The same goes for a site like www.sales-exchange.com vs www.salesexchange.com which might be confused as "sale sex change".
I always liked the example of expertsexchange.com. These are the only situations in which I would prefer I hyphenated domain, though I'd rather rethink the domain entirely as Expert Exchange ALWAYS looks like Expert Sex Change to me now.
Four-square does look nicer to me, but it is not a big difference, because the words' individual meaning is not important. You could say that "foursquare" is simply a new word, and in english text about that company, you would write about "Foursquare", not "Four Square".
On the other hand, experts-exchange is much better than expertsexchange, because the two words carry own meaning---you would not try to create such a word in english but write "Experts' Exchange". Therefore, I believe that these words should be separated. The standard way to separate words in domain names is the hyphen.
It's not necessarily because dash-domains are bad, but out of the top million websites, domains containing a hyphen are less common among the more popular sites. See histogram at http://abznak.com/pub/20101228_dashdomain.png
I'd say if your business depends primarily on search/keyword traffic then hyphens are ok. Otherwise, no, since you are just giving away traffic to the non-hyphenated domain.
Example of search/kw dependent domain would be something like if you sold wedding favors (wedding-favors.com, for example).
it could significantly reduce the value of the site if much of your potential word-of-mouth traffic goes to the non-hyphenated domain first
Germany seems to be the exception in preferring hyphenated domains, which is probably a consequence of trying to avoid multi-word domains being confused with the German language's many clumsy-looking compound words
If you buy "your-site.com" to protect "yoursite.com", which you already own, then it's worth it.