url shorteners use either a 302 or 301 redirect. i think we can all agree that massive 302 redirects can create headaches. as for 301 redirects, they do not pass 100% of the link pop. also multiple redirects can cause issues.
not to mention you are making your main site semi-dependent on another domain, which you might unexpectedly lose control of (see libya taking back certain .ly websites)
personally i probably wouldnt do it. for most sites i do not see enough benefit. i can think of one or two unique situations were this could be a good idea.
according to matt cutts (and my own personal experience) there is some link evaporation going on with 301 redirects. it is not a huge loss. i believe this first became public knowledge when eric enge interviewed matt cutts in march 2010.
I use 'em. Some urls are just too freakin' long for email, twitter, etc. Specially on CMS or Ecommerce packages that tack on lotsa categories.
Right now I'm using a service with one of my own short domains; it is my hope to "roll my own" before too long - there are a number of PHP packages out there, but so far I haven't been able to get one to work on my server. I know it's out there. I just haven't found it yet.
We use our own special shortened URLs e.g. ourdomain.tld/BandBWF which redirects to ourdomain.tld/big-and-blue-widget-foo. It's not on our website but for our brochures, both printed and PDFs to help type-ins and get people straight to the relevant part of our site. Is that what you're talking about Husky?