Are cheap electric brad nail guns worth it for fixing mouldings?
If the question is 'air powered vs. electric' I'd second Chris's comment.

The electric brad nailers, IMHO, aren't useful for finish work. I bought one to do trim work on a room and threw it out after only getting one wall done. They don't have the power or accuracy that the pneumatic options will give you.

Plus, EVERYONE should own a compressor and a pneumatic gun. They just make doing DIY projects that much more fun! ;)

There is a 3rd option, and that's a cordless gun. They are basically a pneumatic gun that has a mini electric compressor built in. These are more expensive, but the main advantage (other than being cordless) is that they are nice for small jobs where you don't want to get out the heavy compressor and wait for it to fill up just to shoot 6 nails.