I've always wanted to use one, but the fact that very few domain hack startups have taken off keeps me away.
last.fm and del.icio.us are the best examples I can come up with of successful startups with domain hacks, and I don't think del.icio.us was ever a huge hit outside of the tech world. Plus they both ended up buying/switching to the dot coms anyway.
Not too viable I'd say. The fun already starts when you need to explain the url.
"Yeah it's Famous. Not normally spelled, but with dot us at the end."
I believe a name should be obvious beyond explanation. If you can't afford the straight dotcom better use an slightly longer url that features the name (like getbootstrap.com).
Especially back when the .ly domains were all the rage, but also now that .io have the same SEO as .com domains.