Rayman has seen its fair share of port. As one of the best-selling PSX games, it's only normal that Ubisoft would try and milk it as much as possible. Aside from the original versions on console and PC, we got a bunch of "improved" versions for PC (how much they were actually improved, however, is up for debate - I vastly prefer the first version). Then later we got Rayman Advance, which had great graphics for the console, but not so much in comparison to the original. And now we got a DSiWare version too, just in case people forgot about the game.
Compared to the GBA version, there's no denying that Rayman on DSi looks a lot better. In fact, it looks pretty similar to the original game, although the aspect ratio has been slightly modified (cutting a small portion of the image on the left and right, which makes some jumps actually a bit more difficult). The shadowy and gradient color design has been reproduced pretty well, and this design also makes the game look decent even when upscaled on a 3DS. On a side note, it would probably look pretty nice on a big-screen DSi XL.
Less of an effort went into the cutscens, which are the same as the GBA version (eg. looping every three frames), possibly in an effort to save space. They also look pretty compressed. Most disappointing is probably the music: while almost all of the tracks are there, the game now selects a single track for every level and loops it, instead of cycling through all the three or four available ones. Some of the tracks were also swapped around for unknown reasons. Finally, it seems the music is particularly low-volume in comparison to the sound effects, and it can't be adjusted in the options.
The core game is the same as before, except even easier than the GBA version (which was easier than the PSX one, which was easier than the PC one...). You start with 6 life points and can increase to 10, instead of 3 and 5. You'll get a 1-up every 50 tings, instead of every 100. Most camera chase sequences have been removed so you can take your sweet time instead of rushing, and some of the most difficult routes have had added platforms. Overall it's much easier than the original game, although compared to most platformers today it could probably still be considered quite difficult: many lives will be lost trying to make difficult jumps. And you can't farm lives by replaying levels. Though you probably won't need it.
Rayman has always been my favorite 2D platformer, so I can't help but want to try every version out there. This DSiWare release looks great but falls a bit flat on the musical side. Which is too bad, considering the original soundtrack was awesome. Otherwise, it's the same game, but dumbed down for a differetn audience. Now, I pride myself on having completed the original PC game with no continues and without a guide, but I'll admit that it would be a difficult sell today, so I won't criticize this choice. And after all, it still offers more challenge than other games today. I guess you can't completely change an existing design. Overall, one to try, if you have never played the original game and want something more accessible.