First, greetings to JB and apoppin. Yes, I still lurk around here now and then.
Second, I've only played through Siberia, but so far NOLF2 is better than NOLF1, in my opinion...
BUT - If you haven't played NOLF1, or many shooters in general, I'd recommend playing NOLF1 first for the following reasons:
1. It's a damn good game (I can hear JB's bile rising...).
2. It's got a lot simpler (and somewhat less satisfying) game mechanics, i.e. no skill level management, less stealth, more linear game play etc.
3. It's more of a straight-up shooter and less of an RPG-Shooter hybrid (this is more like a corollary to item 2 above).
The downers in NOLF1 are GROTESQUELY long cut scenes and so-so graphics (but still pretty).
The downers in NOLF2 are:
1. GROTESQUELY long load times. I can forgive taking a minute to load a NEW level, but a "Quick Load" within the SAME LEVEL should not make me want to run out and buy a 15,000 RPM SCSI RAID array just to get the level back up in less than a minute. Uh uh. No way.
2. The AI can be a little too clairvoyant at times, causing frequent death, which causes me to hit F9, which leads to Item 1 above...
3. The balance between stealth/search and AI aggression can be a little frustrating. I often got whacked while searching in a dark room where I was theoretically concealed. This was due to Item 2 above which leads to Item 1 above...
4. It's a bit of a resource hog. I can run any Quake III based game at 1600x1200 with full detail, trilinear filtering, etc. etc., but I had to step all the way back to 1024x768 with this one. And I still get stutter every now and then. This is with a P4 1.6A @ 2560 and a GF4 4600. But I've heard reports of smooth going on slower systems, so maybe it's just something about my rig.
5. Enemy respawning. Remember SS2 where there were theoretically something like 40 crew members on the Von Braun, but by the end of the game you had probably killed 20 times that many "Hybrids"? Same thing with Russian Soldiers here. Playing on "Easy"seems to control this somewhat, but "Easy" ain't everyone's style.
6. I miss the NOLF1 equipment load-out screen (or maybe I'm just not to the point where you get it yet). The one glaring omission that keeps this game from being a true SS2/Deus Ex RPG/FPS hybrid is inventory management. This can be a drag sometimes, but a Soldier of Fortune-style "load-out at the start of the mission" option at least would be nice.
This is mostly nit picking (except the buzz-killing level load times). Basically play them both and you'll have a damn fine time. Then play Thief 1 and 2, SS2, and Deus Ex if you haven't already.