Having just played through Quake 4, which was released six months after Area 51 in 2005, it'll be easy to draw comparisons in that both themes are modern base.
Quake 4 received it's share of undue criticism, with an aggregate score of 81% on gamerankings.com. Area 51, meanwhile, scored an aggregate of 78% so only marginally behind, but both IGN and Gamespot both rated Area 51 as better than Quake 4.
Nevertheless, it seems to be a fairly obscure title on the PC despite many positive reviews. I asy obscure, because it was eventually released for free (with advertising). But that version crashed on install for me, and I couldn't find a second-hand version to purchase. Even then it took two torrent versions before I finally got a working copy.
The PS2 version was thought to be the best FPS ever at the time, although that's not saying much. This led me to go in expecting a gem of a game. Unfortunately, my expectations were not met.
From the design perspective; both the power of the engine and the level design, Quake 4 eats Area 51 for breakfast. Some of the earlier Area 51 levels look quite bland. Since the entire game takes place in an under Area 51, I guess the designers were trying to maintain some realism in the design. With realism comes bland, futuristic walls and textures. Later in the game levels start to get more variety with the peak being the alien world and UFO ships. However, while the engine is capable of something great, the level design doesn't ever reach awe-inspiring. Good, but not great.
Action which was fairly fun and fast all the way through. The dual-weilding of weapons is fantastic and doubles your fire-power. Having two shotguns go off in quick succession, blowing your enemies away, was a great feeling. However when your down to just one weapon at a time, they feel a bit weak. Unfortunately, it took me most of the game before I realised that the BBG, a powerful gun in it's own right, is self-regenerating.
Along with this, you get the half-man/half-mutant idea also done in Quake 4, where you can transform into mutant and use it's abilities. Every kill you make while in mutant-mode gives you health back. Area 51 definitely does this idea better than Quake 4 and makes it mean something with the gameplay. In Quake 4 you wouldn't know the difference.
Provided you don't overuse the BBG, Area 51 is still quite a challenging game.
Now to the cons: the biggest problem with Area 51 is, firstly, no quicksave. You're forced to use 1-3 checkpoints through each level. Fortunately they're usually in useful places so you don't have to do much replaying, but I still loathe the idea of no quicksave in a fast paced FPS. Perhaps developers think if your forced to replay much of the game then it'll extend the overall playtime. Even so, it's still only barely a 10 hour game.
And secondly, the game would come to a crashing halt at the end of almost every level for me (with a total of 18 levels). With no quicksave, i'd have to replay from the previous checkpoint only for it to crash again. I was forced to use someone elses saved games in order to load each level. However it wasn't such a big deal compared to most games, because your ammo and health in Area 51 are often 100% anyway.
So overall, Area 51 is definitely a decent game to play alongside the best of 2005. Just don't expect anything groundbreaking.