As with some of the other reviews on this site, I must first point out that Jam Sessions is not a game, it is designed as a portable musician's tool. In that respect, most DS owners will NOT enjoy it.
Jam Sessions allows you to strum your DS like an acoustic guitar, using directions on the D-pad to change chords. You have the ability to program the 16 chords on the palette (8 directions, plus 8 more while holding down the L button), allowing you to play virtually any songs that just require simple chord strumming.
While Jam Sessions allows a lot of chord options, there are many chord which could (or should) have been included to make this package feel more complete. The ability to create your own chords by choosing notes is something which I was very suprised to find not included.
The pre-programmed songs on this game, to be honest, aren't that good. The lyrics are not written in time and the MIDI playback is only available on some of the songs. However, this is not really a feature of the "game" that you should be taking into consideration when buying it.
The effects you can add to your virtual guitar really don't sound particularly pleasant, however the natural acoustic tone is very nice.
You are also able to record 5 songs, which is handy if you come up with something good but don't have any other recording options immediately available to you. However, the limited recording space does mean that recording can only really be used as a temporary thing, so that your creations aren't lost before you can transfer them to something else.
This product really shines when you use it to accompany other peoples (or your own) singing, or for creating chord structures on-the-go, but doesn't do a lot else. Jam Sessions does seem quite rushed and not entirely thought-out like a product of this kind should have been. As a result, this is probably something you will specifically want, rather than something that may appeal to you.