I'd pay $2 to trial it, and I'd use it while jogging and listening to music. I have no idea whether it'll work out -- twitter timelines tends to be very fragmentary so I don't know if I'd be able to follow conversations well. However, it just needs to hit that 'cheap enough that I'll buy it anyway' price point for apps.
Trying to be as honest and helpful as possible, I generally prefer to avoid fragmenting features over lots of apps. For instance, the other day I wanted to send >140char tweets, and was annoyed that my main twitter client didn't auto-split tweets. I have a separate app (TweetSplit[1]) which allows me to flip between apps to do the work, but it's not great. The feature (split tweets) was implemented in a separate app and that was annoying.
Reading out tweets seems like a very similar proposition. I don't know if I'd sustainably use one app for reading, and another for TTS. There's friction around logging in to separate apps, switching between apps, and keeping your reading position in both apps.
I think, then, I'd ideally like to see this integrated into my main twitter app. So maybe another group to sell to are developers of twitter clients?
Also worth noting that Abvio's Runmeter app [2] will read out your mentions during a run, which is where I got the idea. I don't use this running app any more, but would still like the feature.
Lastly, maintaining reading position between apps has been looked at by Tweet Marker[3] and might help mitigate the problems of having separate apps for reading the same timeline.
