Hmm, I haven't heard anything bad about it really except they changed a bunch of names when localizing it. I personally haven't touched it because I don't have a DS, but the only thing I noticed when I saw it was I preferred the cartoony art style. Shadow dragon looks a lot more like the older console FE's, and actually has really smooth animation, where in the gba you literally had 3 frame animations with a lot of tricks to make it look convincing.
The system itself is a little quirky. Side chapters you have to intentionally kill off characters (or just be really bad and lose them in the normal course of the game) but it isn't too big of a problem because they just throw characters at you to recruit. There are pretty interesting changes to weapons triangle and weapon skill as well, but the average player doesn't really care about those. The game is heavily rebalanced compared to the original which had horrible growths, but I was under the impression that your units were still very unimpressive, as opposed to the last several games where your characters were pretty much crushing anyone in their path. Javelins are critical because you don't want to be taking enemy counter attacks, lol. All in all, if you are a hardcore FE player, you will probably like it. Game reviewers in general tend to miss the point a lot of the time, complain about the difficulty of the game and in general just don't appreciate the system.
On that note, an FE you probably can't go wrong with is 9 on the gamecube (path of radiance). The hard mode is absolutely perfectly balanced to be fun, but not difficult with the exception of like 3 chapters that just pissed me off. The one problem with it is that the localization team decided to take maniac mode out and put in an easy mode, so you won't get to experience that without imports. Really want to play it though, since I here it is one of the best balanced maniac modes as far as entertainment goes, and up there with FE5 as far as difficulty goes. If its anywhere as fun as Hector Hard mode was in 7, I'm down, lol.