When I played the first mission I though to myself, this is childs stuff. I soon found out I was wrong, very wrong. Ha! Unstoppable Gorg is definitely challenging, moreso than Defense Grid I'd say, mainly because of the limited strategies available. In most cases it seems you need to pick the brains of the developers or read their minds, and find the right strategies that you are supposed to use, which can be frustrating. I much prefer DG's freedom of play where you can still win even if your not going for gold every time, whereas UG is pretty much all or nothing, once your 'castle' gets attacked its pretty much over.
It's fun for a run through, probably won't spend to much more time with it. I made it to the 19th chapter before I needed some help. That was mainly due to my own misunderstanding and misuse of a few towers and abilities though. Recommended.