The TwitterPeek is a dedicated "tweeting" device. That's all it does -- Twitter, nothing else. It's available in two versions, one that comes with lifetime service for $199.95, the other for $100 less with six months of service, after which you pay $7.95 a month.
I don't like it enough to shell out that kind of money myself, mind you, but I do like it enough to put both versions on my Amazon Wish List and hope someone really, really loves me, or at least wants to see more tweets from me.
This device will probably really only catch on with two kinds of people: Those who "live on" Twitter as a matter of personal lifestyle preference, and those who would like to tweet more from out and about (at, say, political events) for micro-blogging or other journalistic purposes, but who want it to be easy.
I'm in the latter category. I have a cheap (prepaid) cell phone w/camera. 90% of the time if I use it, I'm using it to tweet ... and I'd probably do that a lot more if I had something with a real alpha-numeric keyboard on it instead of having to thumb through three- and four-letter combos on a phone pad. Just sayin' ...