I'm an App.net backer... which is to say I'm a subscribing early adopter...
Which is to say, like everyone else, I'm hopefully waiting to see if Dalton Caldwell and co. can really get this show on the road. They set up a Kickstarter-style fundraiser (I don't believe the project would have necessarily met Kickstarter's guidelines as it's not a product but a service - don't quote me on that) and blew their goal of $500,000 to get the ball rolling.
It's not clear to me if this is their target annual subscription, or just seed finance to payroll the development process. However, it seems to my layman's eyes that there is a lot of support for App.net and if it can deliver what is essentially Twitter the service without Twitter the company then a long and healthy operation seems more than achievable. Without Twitter the company, there will be no adds, and with a paywall as deterrent there will hopefully be no spam.
It may sound a bit unfair to say, but with a paywall it may also dissuade a large number of casual users, rekindling something of the strong, somewhat nerdy community that a lot of Twitter old-timers talk so fondly of. Whether this is a nostalgia trip or not, there does seem to be a techy/geeky culture building up on App.net.
They have a year before they start asking me for more of my hard-earned and sadly quite scarce cash, and a determined team can do almost anything in a year. I have high hopes in particular though for third party app development. If I could convince Tapbots to release a client for the iPhone I would be a very happy man.
I won't be leaving twitter, far from it, I have a lot of friends and acquaintances on there that won't be interested in a distinctly similar but paid service. Twitter will remain useful to me, and continue to be a place where I can discuss things, relay interesting articles and promote my interests and my blog. I anticipate App.net offering a more focused and narrow set of discussions and interests, and that's pretty fine by me.