I run several Twitter accounts with lots of followers (in the hundreds of thousands). On some accounts, I tweet once a day, and on others I tweet 20+ times a day. I've tried timely.is, bufferapp.com, SocialOomph, TweetDeck, Hootsuite, and a bunch of other Twitter queue sites.
I really like the design of yours. And I really like how you can bulk buffer tweets—the only other site I know that can do that is SocialOomph, and it's HORRIBLY designed (seriously, check it out.) But why not give the option to type the tweets in on your website rather than email? The emailing seems like a step that's convenient in some cases, but gets in the way when I just want to paste a list of tweets in.
Also, I read your FAQ so I know it's coming, but a customizable number of tweets per day would be great. Also, ability to link to multiple Twitter accounts!
I'll give buffero.me a try, and if I think of any other features/critique, will let you know here!