I know this seems counter intuitive for someone who's providing a URL shortening service, but:
If you don't absolutely need to, follow joshua schachter's suggestion and don't use a shortening service!
In most cases it's really creating more trouble than it helps. If you don't absolutely have to, just use the real URL - not a shortened one.
Of course for the cases where it does help (particularly bad long URLs that break in emails, twitter, IRC, to give over the phone, ...) I do encourage you to use Metamark.
I'll give you just three reasons:
* We're committed to the long term. One of the reasons we haven't added as many features as many other sites is because we're here for the long haul and want to make sure it's something we can support not just fro the next months, but for many many years.
* We use 301 (permanent) redirects. No funny frames. No "temporary" redirects. This means that Google etc are more likely to "give weight" appropriately to the target page you are linking to.
* Basic stats, but reliable service. A often requested feature is more sophisticated stats; and we will get to it eventually, but our main focus is running the service reliably and "forever". If we ever shutdown the service what we will do is turn of creation of new links, but make sure all the old short links continue to work.
But again - best of all is to just use the correct URL; not a shortened/munged one.