So, anyone who's played it already: what's the expansion like?
I already have the original release of Painkiller, so I'm interested in finding out what the expansion is like in order to decide whether to buy the GOG edition as well or not...

Its a classic "more of the same", unfortunally someone else did the CGI scenes in the expansion and they are not nearly as good as the ones in the original game.

Expansion?
In short: Pretty fun.
In long: The polish and level design feels a bit weaker in parts, but stronger in other areas. It's the same Painkiller you've been playing, so still good fun. There is a couple of new weapons to IIRC.
The CGI cut scenes are rather "blah!". But, it's such a small part of the game that it does not take it down any way. Unless, you are a big fan of cut scenes.

The cutscenes in Painkiller were always pretty underwhelming tbh, thankfully Battle out of Hell doesn't add too many more of them.
The new levels look really good and are mostly well designed. They're large with nice texturing and very good lighting effects, the letdown being that there are a bit too few of them. The new monsters are well modeled, and generally more difficult. They're still dumb as a pile of rocks but they tend to go down harder.
Gameplay is same old same old, just run around in circles while firing. Shallow but surprisingly entertaining. The exaggerated ragdoll physics is still there and still fun to watch.
However there are a couple of just stupendously hard boss battles (doesn't help that it's pretty counter-intuitive how to beat a couple of them), and some pretty tough jumping puzzles as well.
Overall it's a bit below Painkiller in quality, while the levels are better and larger, there are a few chokepoints where you will probably do some hefty reloading to get through, such as a couple of boss battles and level specials. Doesn't sound too bad, but the loading times are still pretty bad in this one, and reloading after dying for the mpth time 30 seconds into a boss battle can get frustrating. A bit too few levels and new content overall to merit a full-priced purchase back when it was released, but good value now.
If Painkiller Original is 8/10 the Battle out of Hell is a 7.