I'm considering getting this game as it does look rather fun.
However, I've read that there are only two islands and that once rock is dug out for rooms it can't be filled back in. So, my question is, with only two islands and base layout that cannot be altered, only added to, does this affect the longevity of the game? Can you only have two bases throughout the course of the game?
Also, do you unlock new technologies for traps which gives a reason to replay it? Is the second island effectively an unlock so that once you get there you can have many saves for different bases on that map?
I've read that there is no real sandbox mode, so I'm hoping that something gives it a bit of freedom for replayability.

you have to make choices in the game, so you can play it again a lot of times.
(which of the 3 geniuses, which second henceman to recruit, base layout. location of traps, types of traps.)

Provided you like that kind of game (I do), longevity isn't an issue. The campaign, even if it's the only game mode, is huge. You have only two islands, true, but a lot of things to do and a lot of freedom during the campaign. For example, each enemy super agent has his/her own way to die : you have to discover them all. Research also requires a lot of patience, since it's not linear but works with a trial/failure system (you have to try different combinations of items to advance. If you're lucky, your research goes fast ; if you aren't, it stalls).
Above all, you'll spend many hours trying to device fiendish traps. The trap system is nowhere near Dungeon Keeper. Here, it's incredibly precise. You can make combinations of traps : the guy walks here, then bounces here, then is set on fire because he passes near the flamethrower, then falls into the piranha tank... The possibilities are infinite. Actually, to many extents, Evil Genius is a puzzle game.
It is actually possible to undo/redo parts of your base. Only external structures, like the hotel that is your cover, are irremovable once you've built them. For the rest, as research advances, you may want to redesign your base to take advantage of the new possibility.
There are also three mains characters and various henchmen, but the campaign remains the same, so the longevity isn't here. It's in the malevolent, fiendish, nefarious traps that you can imagine.
A few remarks. If you've played Startopia, you know what kind of boring chore it was to manage your employees. Here, everything is automated. You say how many of each kind of guys you want, and they train all by themselves. What a relief ! Secondly, the overall design of the game is outstanding. The b-movie atmosphere is well done, the animations are hilarious, the characters are cartoon-ish enough so you don't take the whole thing too seriously. Last remark : Evil Genius is a very slow paced game. That's why some people didn't like it when it was first released. Things aren't done immediately : you give your orders, and then your minions execute them -- like in Tropico or the old Settlers games. You don't even have the "magic hand" of Dungeon Keeper, or the teleporter of Startopia. Oddly enough, it brings a little bit of realism in a otherwise farcical game. It means you have to plan ahead, slowly, what will be the mechanics of your base, and hope that everything will go as planed -- even if you know that it won't.
Well, that's a lot of praises (I'm ordinary player, I swear !). I think the story of this game is unfair -- poor press, hectic commercial policies. It deserved a better fate. In my opinion, it is much better than Dungeon Keeper, funnier, more complicated and more precise in all kind of ways.
And frankly, at 9.99 $, if you're not afraid of slow paced, puzzle games, it is a bargain. And be sure to download the free, unreleased bonus CD that you still find here and there.

You can demolish everything, including the hallways. You'll end up trapping a lot of your minions though, unless you give them extra ways out. :)
The two island limit doesn't really come into play too much. There's really not much call for more space once you get established.
There are different endgame movies, but gameplay is the same regardless of which avatars and world domination plans you choose. I was a bit disappointed in that. Still a fun game though. It keeps calling me back.