If you enjoyed Behind Enemy Lines you'll enjoy this one too, as it's pretty much a mission pack for that game with a few nice changes - Notably the Spy being able to take uniforms from disabled guards being the best gameplay change. It's also nice that they added an easy difficulty option for those that want a less populated/merciless map, but you will want to play on hard if you want the game to mimic BEL.
The biggest change that I was not the biggest fan of was the different voices from the original, but they do grow on you after a few missions. All in all a great standalone expansion to an absolute classic Stealth Tactical game. It's a little shorter, with 8 missions instead of 12 (people who are better at puzzles could probably beat this in a few hours as opposed to the 44 it took me). I remember loving this game so much as a kid but I absolutely never beat it back then because only like 2 levels I remember from way back then.
Great game that you pretty much know what to expect if you played Behind Enemy Lines. If you loved that one you'll love this one.