Well, they are very similar, but also very different. My brother and I both enjoy Borderlands very much, and he loves fallout 3 and New Vegas as well but I can't stand them. Borderlands is a very quick pace run and gun kind of game (in most cases) where as Fallout is a little bit slower and has that automated aiming sytem (forget what its called). You obviously know what fallout is like so I will just describe how borderlands works a little bit.
Borderlands:
Supports Run and gun play style, but also sniping as well
Instead of different stats you only have proficiencies which improve your ability with each gun (these improve over time naturally rather than through selection)
Has skills, most are passive and one main action skill, but of course alter your play style
Has four different classes which have different skills
Supports multiplayer playing which is one of the best aspects of the game
No lockpicking or decoding things, you have an action command but the extent of that is "Open" and its done fairly quickly, there is no loading between opening each and every door, only between large areas.
There is very little (emphasis on very) going around gathering clues about past events or history. Borderlands has less storyline then gameplay so if your looking for storyline its best too look somewhere else.
There are massive amounts of guns, since it has a random weapon generator that can piece together almost any combination with different stats.
Can sometimes seem repetive since most missions are to kill a certain objective or to gather enough of a certain item, but that is standard in most games. If you enjoy leveling and improving your gear/stats as much as possible then you will enjoy this game. Thats why it is often compared to Diablo is because the main objective of that after beating the main storyline is to level up and improve your gear as much as possible. (Although that is more of a dungeon crawl with swords and armor and borderlands is much more open with guns as previously described).
Hope this helps.
