Very under-rated game, a true review within.
Now on to the review from someone who can run the game flawlessly for hours on end without a single bug or crash, with every setting on, and on their highest settings...
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Firstly, I think they are really trying to step up and innovate the tactical shooter genre, while still making it different then the new Ghost Recon Advanced Warfighter series. It adds a bit of fantasy, with the very sweet blind firing from behind walls, taking cover, and the oh so sweet upside down rope hanging shooting in windows action.
Even on the highest settings the game doesn't look quite as good as some of the other games, it is definitely designed for the consoles, because there are plenty of PC titles out now that look better. But the graphics do suffice, and keep the immersion of the game in tact. The immersion is however limited, and the levels aren't as free-roam as the Ghost Recon sister title. Which is refreshing to an extent. After playing games like Elder Scrolls, GRAW, and other next-gen titles that thrive on giving players a huge environment to do what they wish with, its nice to have a bit more of a guided direction. But if you're looking for the immersion of Warfighter then you will not find it. The Team-AI isn't genius, but its an improvement from the blithering idiot teammates you have to deal with in the first GRAW, and due to the well put together tactic system, if you plan things right, you shouldn't have too big of a problem. The GUI is a tad bit confusing at first, but the tutorial in the first few levels, if you pay attention to such, should cover all of the basics. Just play with the light on for the first few hours, because you'll be looking down at your keyboard a bit until you get the hang of things. Those that lack the virtue of patience, should stay away from this game, because it will take a while for you to get to the point where you can plan out your iniltration of areas at first.
I do agree that the tactics feel of the Ghost Recon/Rainbow 6 series have been tainted for a while. Concentrating a bit too much on gadgetry and technology, they got away from the true essence of the first two Rainbow Six titles. However, in this one I think they tried to get back to it. However, I think a lot of the criticizing this aspect comes from the lack of understanding. Don't judge it by the first few levels, you're in a very spread out area and tactics are hugely different. It will remind you of most of the levels of GRAW because you're in an urban area of Mexico. You won't get into that close quarters tactics of the old R6 games until you head to Vegas.
Overall I think the game has its flaws, such as the AI is still a bit dull, though it has been improved since GRAW, the graphics could use a boost on the PC release for those of us with "decent" rigs. If you can't play GRAW 1 then don't even bother with this title, and save the money you'd spend for it on a decent GPU. Just because you can run Source doesn't mean you have a good system. Keep such in mind in the future.
As far as I can see the game is pretty clean, I haven't ran into any crashes, or technical problems as of yet. Since you can do many different things the controls are different, but nothing that play-time can't fix. Overall I think they really improved, and are on their way back to having the best tactical shooter title around with the Rainbow 6 franchise. I recommend you play past the Mexico levels before giving this game a review. Vegas will eat up your memory with the environments being so glittery, but if you have a truely decent system you should be able to run it fine. Ubisoft has great customer support so if you run into hardware conflicts it shouldn't be too big of a deal to get it running. The truely biggest problem I found, and I've found it with a lot of newer games, is the sound code being truely inconsistent. Sometimes you can hear gun fire right, othertimes you have to turn so that its behind you before you can hear it. This shouldn't be a problem for console runners, but for those of you running surround sound setups on your PC, you may need to look for some tweaks and fixes and such for the sound to truely be optimal.