Borderlands was a great diversion while I wait for Call of Duty Modern Warfare 2, but its strength is in its multiplayer and once newer games come out, very few people will be playing this game.
Borderlands is in no way as strong as Fallout 3 because Fallout 3 had a deep, immersive single player that was more interesting and allowed for more exploration. Borderlands is divided into much smaller maps that don't quite add up to Fallout's open ended feel.
In fact, Borderlands feels alot like Fallout, but with the focus on First Person Shooting, rather than 3rd person action.
The best way to play through the game is to play online with friends, but even then, its best to level up your character BEFORE playing through the singleplayer/coop. This way you'll be a higher level than every enemy you fight. Many of the game's firefights can be overwhelming and even if you are levels higher than the CPU enemies, their onslaught can kill you quick. If you are right at the same level as the enemy, you'll have some very long and frantic firefights but you'll most likely be frustrated if you don't have buddies playing with you online.
Once you've leveled up your first character and played through the gameyou'll probably not want to do it again. The ending is WEAK. Its anticlimactic and could have been alot better.
The weapons are fun to experiment with but the weapons you'll want most are the alien guns which strangely enough do not really do much more damage than their human counterparts. As GR's review mentioned, sometimes you'll get great new weapons and other times you'll just get crap. The "carrot on the stick" that keeps you playing is the hope you'll find either an alien weapon or a really powerful replacement for your shotgun, assault rifle or rocket launcher.
And then there are the cheats.
The easiest way to cheat is to join a game with someone who is much higher in level than you are. The higher their level the more XP, money and power up you gain as they play. You are too weak to hurt enemies on their level, but you;ll level up quickly without doing anything. The temptation to cheat in this manner is STRONG since it will not only benefit you in duels with other players, it will help you fight through the single player easily.
There is a glitch in the game which allows people to "copy" their weapons and give them away. Its not a very big deal if you don't want them, but once you have more powerful weapons and the level required to use them, you can breeze through the game. The Alien Shotgun for example is a BEAST that kills most enemies in 2 or 3 shots.
When COD 4 MW2 comes out next week, this game is getting shelved. It was fun, but it just doesn't have the replay value I'd hoped it would. There is no open ended feel like WoW has and the game degenerates into waypoint finding rather than simple exploration. This game needs more carrots on the stick to keep you playing past the achievement you'll unlock for reaching Level 50.