Due to the fact that you have to either assume users already have SL4A installed on their systems, or package it with your app, it's certainly not an ideal solution for developing a typical Android Market app. There are definitely a lot of specialized applications that it is well suited to address though, and it can also be useful to quickly prototype a tech-demo for an app that will later be fleshed out in Java or one of the NDK-supported languages.
Prototypes, sure, but people always mention it to say “You can write apps in Python on Android”. That’s simply not true.
SL4A is kinda like PhoneGap: you can use it, but you will not get a quality app out of it. That might be fine for your use case, but it’s definitely not on par to using the canonical platform.
Other quick notes: The last commit on the Scala for Android project is from two years ago and says “add 2.2 support”, the related blog post is from one year ago. Same deal with Android ObjC: last commit April 2010. I would never trust either of these for production development.
I’m sorry but languages other than Java and C++ for Android development are closer to vaporware than reality right now. The only good alternative is Mono http://xamarin.com/monoforandroid which is actually working and supported and not a forgotten weekend hack.
