For all intents and purposes, this is a guitar simulator. At no point will you feel like a Guitar Hero - instead you become a struggling musician who can't ever seem to get a decent gig. It isn't really a game, with absolutely no set challenges and no direction to guide you on the way to success. Even then, success isn't measured, so you literally become a faux guitar master on the DS, and have nothing to show for it. Much like a real musical jam with friends in some garage has no set purpose, neither does Jam Sessions. Akin to lazing back in a chair with an acoustic guitar and strumming along to yourself, Jam Sessions gets boring pretty quickly, and is far less exciting that actually playing a real guitar.
To play chords, the D-pad (or face buttons for southpaws) must be pressed in any of the standard eight directions. While held down, the touch screen can be "strummed" with the stylus, moving across the horizontal line in the middle of the screen. Change the D-pad direction to change chords, and the L button can be held to switch between different palettes of chords. Depending on how you move the stylus, the guitar sounds will respond accordingly, whether hard or soft, fast or slow. The mechanics do work well, but that's almost like saying if the wheels of a car turn, then it must be a good car. Jam Sessions is not a good car. It's not even a good motorbike.
A severely limited selection of play modes are available, if playing is the appropriate word. It's best to start off in the Warm Up and Tutorial modes, playing very guitar-intensive songs like Coldplay's Yellow. Long story short, you'll be strumming and sighing with boredom. The actual interface is pretty easy to use, with a metronome ticking away to keep the beat, and the on-screen indicators describing either an up or down stroke and length of tone.
Beyond the pre-set chords for each song, you can manipulate the chord palettes at will. I began to drag and drop different chords to different D-pad selections (including the secondary palette which becomes active while holding the L button), and started experimenting with the different tones. That's when the problems began.
The box plainly states that if you aren't a musician, you'll still be able to enjoy Jam Sessions thanks to "comprehensive tutorials". Well, the tutorials aren't comprehensive. Playing chords is fine, but each chord is labelled with a note and some kind of symbol. Fair enough, I assume that these are various musical symbols - but there lies the problem. The game never teaches you the relevance or use for any of these symbols, or the genuinely musical features, and anyone who is already musically inclined would hardly consider playing guitar on their DS - they would play the real thing. Gn7? Bm7/5? What the fuck is that!? Guitar Hero makes playing cool songs easy, and Jam Session makes playing lame songs awkward.

To add to the confusion, a number of options are available to tweak your musical enjoyment. Tweaking certain things is almost a one-way track to instant enjoyment, but I have no patience to tweak my DS/Guitar to use Option 4 - Low Cut Distortion with a side of Flanger. Would I like fries with that? Only musical fries, the kind that play "Komm, Susser Tod" as I eat them.
Basically, it's a mess of a musical menagerie. I can understand that all this stuff makes perfect sense to lots of people, but it made no sense to me, and Jam Sessions made no attempt to help me out as I fumbled hopelessly with the poorly annotated interface. I spent most of my time picking a different coloured background skin from a rather massive collection of stylish art pieces and unique colour combinations. Note to developers - when making a "game" based solely on the digitalisation of musical prowess, art direction is not a priority!
The (lame) song selection is limited to a few low-profile tracks from an even fewer selection of high-profile artists. While notables like Beck and the Jackson 5 show up, there's a limit to how long you can play the digital variations of B-sides. The rest of the tunes in the track list aren't even in English. Attention all Australian gamers! Ever wanted to play Centro Di Gravita Permanente by Battiato on your virtual acoustic guitar? How about a rousing rendition of Irgendie Irgendwo Irgendwann by Nena? Those classics and more are available right now, thanks to Jam Sessions!
I've got no problems with international music (seriously, get Komm, Susser Tod on there and I'll be an instant fan). In fact, even the stagnant Guitar Hero series could use a burst of worldy creativity beyond the narrow-minded metal fetishes that the chaps in charge seem to have. However, adding a bunch of somewhat inaccessible songs to a game with an already casually inaccessible interface is a recipe for disaster. Sure, this version is probably being churned out for markets all over the world, and good for the Germans for giving them what they want, but to use the same, already underdeveloped game in all world markets is just lazy.
That's all there is to it. there's no passing, failing or screwing up, and you can play any song at your leisure. You won't be penalised for playing an incorrect chord, or even playing too mahy. The song chart simply scrolls down the page as you play a rough version of the chords and stumble through. You can play the opening few chords, put the DS down and scratch your balls, then pick it up again and keep playing. Again, this isn't really a game, only a virtual guitar. Casual gamers would be turned away due to the inaccessibility, and musicians would never think to pick up an inferior version of an instrument with which they are already skilled.
I was the same as you, once upon a time. The concept looked cool, even the gameplay videos sparked some interest - but all time spent becoming associated with this "game" is wasted. There's no reason at all to give Jam Sessions even a passing thought.
Oh, and you can sing to yourself through the microphone. Cool, huh?