Yes, Last Stand Arnhem is latest.
Comments added below your questions;
1) Is it any good? Are there many bugs, crashes? Is the AI painfully retarded?
- LSA is good, though with the number of changes made to it over CC5/WAR/TLD it has been a bit buggy. On the good news front patches are still coming out and it is getting better each time.
- The AI question is hard to answer. I find it fun to play against but there are those who say all versions of CC have a useless AI. The truth is probably in the middle as the AI can be at either end of the spectrum depending on the battle.
2) How expandable is it? Can you add maps, units, campaigns etc?
- The game is mod maker friendly but is not easy to change within the game. You must edit text files and have graphic skills to make anything more than battles/operations with the current BG's and maps.
3) What nationality units are in game. Is it just British/Germans and possibly USA/Poles/Italians too? Are Russian units available?
- British, Dutch on both sides, US and German
4) Air power - how does it work? Worth seeing?
- The amount of air support is historical with a bit of a random factor. When available you can assign air support to specific BG's and then in battle, if a plane shows, you then target an area to be attacked with smoke.