An answer would be dependent on knowing what type of application because no single user knows all parts of Mathematica. My own interest is learning some modern mathematics and physics, writing Applications, and trying to learn how to best use Mathematica as a technical communications medium.
I don't see the bugs or "features" in Version 10 as being more or worse than in previous versions, and on the positive side there is a lot to like. I wish the principle core user interface would settle down and stabilize a bit more than it has. I'm not too fond of gratuitous changes to the core cascade style sheets because Application style sheets are built on them. I don't like it when useful features such as single word spell checking (or word completion) in Text cells is taken away, because it was very useful to people writing notebooks as communication documents.
I certainly will use Version 10 for my present Application work and the chances are it would be a good choice for your own work.