Youssou N'Dour has since the 80s tried to popularize his music outside of Senegal, so he started making albums every couple of years geared towards the Western ear, which is not acquainted with the complex Sabar rythms. For that reason he used to turn down the percussive force on those albums and added softer, synthesizer infused sounds. Also he started to add songs in English, wich however takes away from the amazing voice he has when he sings in his native Wolof.
He's had some success with it though, after all his most famous song outside of Senegal is "7 Seconds" with Neneh Cherry.

However, he has always recorded pure Mbalax albums that everybody in Senegal loves and that more and more people outside started to seek out and appreciate. However his albums recorded for the local market are usually not published in the West (such as the outstanding Lii!).

Fortunately after those synthie-polluted albums of the late 80's and early 90's his newer albums that were geared towards the international market consist of mainly pure Mbalax with the token collaboration in order to make Westerners take notice of him. Don't judge him by those collaborations, they're usually simply bad.

I would recommend Set, Immigres, but then skip to Joko, Nothing's in Vain and Rokku Mi Rokka which are all from the 2000's. Egypt stands out, it being a collaboration with an Egyptian symphony orchestra highlighting his Muslim belief and deep spirituality. Finally, his latest Dakar-Kingston is especially great for its remakes of some of his hits in a Reggae style. A few songs are cheesy but I highly recommend listening to Leteuma, Medina, Bamba, and Africa Dream Again vis-a-vis their original versions. The songs are easy to find next to each other in his complete song list.

Update:
You can watch an entire concert of Youssou N'Dour from the Sacred Music Festival in Féz, Morocco from June 16th, 2011 in high quality on Arte TV. Highly recommended!!!