From a "Client Framework" perspective...

I don't quite see how drag & drop could be useful in a form. But that's probably me!

It takes some time to get used to the way Backbase works, in my experience. And it can be less time consuming, but it doesn't have to be. The client side validations methods are fantastic and make it easy to create a interface that's intuitive from users perspective. The extra option to validate client side does make your application safer en nicer to work with. But you'll most likely want to do a server side validation to. So if you weren't already maintaining a server- and client side validated form, you probably won't gain any time in maintaining the application. But if you're using any kind of client side validation, you wil for sure (after investing a few day's in getting to know BB).
Whatever you're current situation is, you'll most likely end up with a more user friendly and safer application.

I have only limited experience with BB. But so far these are my observations..

Downsides...
- Backbase slows down your application for sure. Loading times will increase almost certainly.
- Getting used to BB takes a while, depending on what you're used to.
- The different approaches to the same goal can be confusing (you'll know what I means once you get started with BB).
- The reference is very useful and helps a lot. But there is serious shortage of step-by-step tutorials for some specific and obvious solutions.
- The framework is in development. I would mark some features as 'beta'. (some people might not agree...)

Upsides...
- You will be (somewhat) forced to work in a Model View Controller way. Which is a very (very, very...) positive aspect (again... I have no experience with the JAVA version).
- Service is great. Even if you are using the free version (2 CPU's 1 server). Everybody is very helpful! The framework is growing more rapidly then the community is though.
- You're application will be as safe and cross browser capable as it can get using Ajax and client side validation.
- You'll have so many easily accessible eye candy and really usable features you'll probably get inspired to completely rewrite your application :) If not,... to write a new application.

For short...

With this framework you'll have the ability to write a application with which you can impress your boss and colleagues for sure. It will be easier for you to maintain and easier for the clients to use. But... before you'll get to that point you'll have to make a (serious) investment. Personally.... the + is bigger then the -. But I had to say goodbye to BB three times (and come back, because of the promising demos) before I learned how much fun BB is and that it's worth spending a lot of hours/days of my free time on (to get to learn it). If you know Javascript and strict XHTML well you'll have a big advantage.

My 2 cents :)
Good luck with your choice.