Definitely try before you buy, but it is a really fun game if you like the 'build your own stuff' kind of thing. If you're more into storylines, you probably will find it a bit lacking. Infinity is a game that wants you to make your own fun.
You can try it out by downloading the free pc-version--I think it comes with one character to use? The controls are really different but the idea's the same.
I have all three, and most of the characters from 1.0 and 2.0, so I can tell you the compatibility basically works forward, but not backwards. All the figures will work on any later system, but no new figure will work on a previous version. And the more recent the figure the cooler its abilities, generally.
The figures can be leveled up over time, earning new powers, but they have it rigged so that you can't have *all* the powers available--you have to choose. You do get two chances to reset those choices tho', and it doesn't feel restrictive at all, usually you have no trouble getting everything you want.
As for the Toybox, that's where the real fun is--any character will work, and the toys you unlock (from playsets or the shop) all work. You can build ENORMOUS toyboxes if you have a good enough console. (My little xbox 360 strains a tad, lol.)
The toybox is the best part of the game for a lot of players, it gives you room to just goof around and have fun--it's way better with two players, though.
Just imagine playing paintball all over Cinderella's Castle, while dodging giant laser-shooting robots of death, and riding Sven from Frozen. Meanwhile your opponent has totally booby-trapped the heck out of *everything*, and is invisibly sniping at you from the tallest tower.
But that's okay! Just plop down a Sky Changing Machine, and turn out the lights! Let's see her hit you *then*, hah!
...Totally random scenario, I swear.
As you can see, I like the game a lot. It is tons of fun if you have kids around, or people who are able to act like them, lol. (But it can be kinda lonely playing alone.) I hear the newest one has a matchmaking feature to help you find people to play with online, but I prefer to play with people in person. (The trash-talk gets pretty hilarious.)
Right now, the best way to get the most for your money is to get 3.0 on sale, and add figures as they're available. (Best buy, target and TRU frequently do cheapie sales online for individual figures and certain versions only.)
The best reason to do it this way is that each new game version includes *all* the unlockable toys from the previous versions, thus: go for the latest, and get more things to play with! This includes toys earned from playsets, special promos and power discs! It's great.
One really neat feature lets you play with toys in a toybox even if you don't have them unlocked, so when you play the user-created toyboxes, you can see all the neat stuff out there to find.
Oh, one major drawback is that the playsets are version specific, so you can't use them unless you have the right game version. ( The lone ranger is my favorite by far, and I have to break out the 1.0 disc to use it, *sigh*)
The biggest con for the whole game is the price--you can spend a lot of money on figures, so you want to be sure you're going to get a lot of use out of it. But if you pay attention to sales and such, you can get some really good deals.
Infinityinquirer.com is a really handy website for keeping track of news and sales.
Hope some of that helped!