I've played all sorts of single player FPS and RTS games, yet none so far have me gripping my keyboard and mouse, often jaw dropping in suspense like Jagged Alliance 2. Something about the combat, intensity, and diversity of the game have me hooked. I'm playing JA 2 with the 1.13 user made patch.
Are there other games like this or is JA2 unique?
Thanks. :)

Yes they are.
Check out Silent Storm. It is very similar to JA2 and it is very good too.

+1 for Silent Storm. Quick synopsis : You lead a spec-ops squad in WW2 against the Germans. Starts off as a standard squad combat title, but soon becomes...interesting (laser rifles, Panzerklein mech suits, etc.). Really well done game, though, with a nice combat engine.

Haven't played much of JA2 or Silent Storm (yet, anyway), but suspense and diversity makes UFO: Enemy Unknown AKA X-COM: UFO Defense a pretty safe bet. Terror Missions are very aptly named.

Just be warned that the Silent Storm interface feels amazingly clunky compared to JA2. It is indeed a great game, but if you play it so soon after JA2, you'll probably get frustrated. I know I did.

I can't seem to find where to buy Silent Storm Gold. Any suggestions?
Any chance of GOG carrying it? :)

Silent Storm lacks JA2's strategic layer, but is a very good game.
The JA2 experience (great turn-based tactical combat, varied weaponry, compelling characters, sense of humour, resource management/logistics and strategic freedom) is unique AFAIK. Plenty of other games come close and include most of these, but JA2 is the only game I know that has them all.

The only game that has all of those things that I can think of (aside from JA2) is 101st Airborne Over Normandy.

For me the gold standard in turn based strategy is still Blue Byte's Incubation, but I'm not sure it's what you're looking for as I have never played JA... maybe somebody who has played both can comment?