Well, I finished this game 10min ago. Took me about 2 weeks, on and off, on medium difficulty. I used the unofficial patch FCAM 1.5 which solves the "mercs see through tents" issue, and improves textures (with bumpmapping and other cool stuff).
I must say I enjoyed this game, especially the jungle/mercs levels. The stealth aspect is pretty fun, and implies some strategic planning before running into the fight. Besides, the game aged very well, and still looks very good. And it runs perfectly on today's hardware.
I had read that the game was difficult, and indeed I'm glad I only selected medium difficulty for my first playthrough. Even though I consider myself as a decent FPS player, some levels are a pain, especially the last two. I had to resort to online walkthroughs several times, as I was sick and tired of reloading the last checkpoint, playing 20s, getting killed because I did not know what to do, and so on... speaking of which : the checkpoint system is fine, but I could have used a quicksave button too...
Overall, I think this game is definitely worth the few bucks that GOG is charging for it. However, the single player campaign would have been so much better with some more playtesting to get rid of some unbalanced sections !

This game is a pure FPS. With two tedious sections. They're short but it's never fun to die when you can't really do anything about it.
For anyone that wants a challenge, play on the hardest. It's more fun.

I'm new to the FPS genre, and the aircraft carrier section in the beginning is kicking my butt. I've played it over 15 times now, the last few on easy. I just cant beat it. :(

I just finished the game, too. I had some trouble in certain areas, as well. Like you describe. But as I wrote in a different thread, I was able to play through the game (without walkthrough or quicksave) by changing my tactics. In some places being a hunter/stalker works great. In other places this just doesn't work, you need to be more aggressive and go in guns blazing. Otherwise you're just overwhelmed. And again in other places you just have to RUN! :)

Thanks for the tip ! I'll use it when I replay the game or if I play a mod.