Twitter seemed really fun and cool at first, and Twitterific was a brilliant free program that made it that much better. And then they decided to start charging for it...which is fair enough, of course, as they spend time developing it, but $15 is simply a rip off for this very basic program. I would have gladly paid $5 or maybe even $7 for this, but $15 is just over the top for one little window that can't even remember its contents between launches and makes me keep marking the same messages read everytime I start it up. And to me, there's no point in even using Twitter if I have to keep going to a webpage to check it every time. Net result: I simply stopped using Twitter.
I think it could have really taken off if this great client remained free and was cross platform, but I find the ads irritating, was annoyed that I had to keep marking the same messages read everytime, and found it surprisingly slow to start up for such a simple little app, adding that much more of a delay between when I log in to my computer, and when I can start actually doing anything (several other programs also launch at login, and Twitterific's surprisingly slow startup time added a not-insignificant extra delay to the process).
At some point this client stopped being about bringing something new and cool to everyone, and the focus was shifted to making money rather than providing a quality product. That's when it started to go downhill. Granted, the actual Twitter service itself has not become commercial, but like I said, to me it's useless if I have to keep checking a webpage; it really needs a decent client, and sadly, Twitterific stopped being decent right around the same time it started costing way too much money.
Quite frankly it's not that much of a disappointment anyway; over the year or so that I used Twitter, it slowly morphed from timely quotes and tidbits and links to cool things, to a boring play-by-play of the most mundane aspects of people's lives (e.g. "Just got out of bed. Time for coffee." or "Dropping the kids off at school." ... good lord, who cares?!). I think we lost the point of this somewhere. But this is just one person's experience.