Any learning you do is invaluable, whether it's in DBPro, UDK, Blender, Photoshop, anything at all to do with anything you want to do.
Someone with experience will take to new systems a lot easier - if you know the groundwork involved in a FPS engine for instance, then it's much easier to learn, even just knowing what is expected of the developer in these engines can be a bonus.
It all stems back to how you view your projects. I don't see projects in one particular system anymore, because DBPro on my laptop is different to DBPro on my desktop is different to DBPro on Netbooks is different to UDK, Unity, Leadwerks...
You can go on for ever and ever, flip-flopping between engines - OR - you can learn to make games. Making games is making games, there are more migratory techniques than most people would expect. The very very best UDK users out there understand why and how it does what it does - someone picking up UDK will not understand it, they will follow tutorials that show them how to do exactly what they are told. The more experience, the more you undertand, the more you can do, the more you can push your engine.
Who is to say that UDK is any good for a game besides an FPS though. I will ban anyone who mentions that bloody underwater 2D thing - it is still made by a company of people. For a soloist - UDK is development suicide. Really, can anyone name a solo UDK project that got finished and released?
At the end of the day, decide on the engine that suits your skill level and game idea - if you are skilled with modelling and design, and know the basics of 3D programming, and want to make a FPS game then UDK might be worth a look.
For every other type of project and level of user knowledge - well they already found the right place (here).
