The best thing I can say about this movie is that it made me laugh. Some of the jokes and scenes were easily on level with the first two movies. Add to that healthy chemistry between the two leads and you get all the ingredients needed for a good buddy-cop movie.
Unfortunately, Besson simply fails to string these bits and pieces into a coherent whole. Bad guys are the repeat from the second part, only without kung-fu and cool cars (pretty stupid decision for a movie targeting car enthusiast), and their plans and actions are complete nonsense. The whole plot seems to be there just as an excuse to throw in a bunch of obvious self-references, lame cameos (Sylvester Stalone as a secret agent?) and extreme sport mix-tapes (Besson's latest fetish, it seems).
Action scenes are the final nail on the coffin. The Taxi series never had really spectacular car chases (the closest they came was the Paris scene in Taxi II), but they at least HAD some chases. Here, it's all bikes, monster-trucks, roller-skates and skis. Daniel's taxi performs several stunts and grows a few more SF accessories, but it doesn't really have much to work with, given the opposing arsenal.
Overall, the movie has some funny bits (even more if you speak French), but the rest of it was pretty disappointing.