I don't have a huge amount of experience with them but I was lucky enough to get one of these for Christmas. After looking around I decided I wanted the Line 6 JM4, which is effectively the Spider Jam amp with a looper built in and the amp removed. The reasons I prefered it over the other more traditional loopers are as follows:
-It has Line 6 amp simulation and effects built into it so you can output to a PA system rather than having to go through a proper amp (ie I can use headphones or a hifi system with both electric and acoustic guitars). It's also got artist presets, amp presets etc like you get on lots of other Line 6 stuff
-It has a big selection of drum loops and jam tracks built into it.
-It is an excellent practice tool. You can change the tempo of anything played into it without adjusting the pitch, or adjust the pitch without adjusting the tempo, and the pitch is adjusted in half-steps so you can't move it "out of tune". This includes being able to record whole tracks off your ipod on to the looper, or putting wave tracks on a SD card and putting them in the looper directly. This feature is absolute genius for learning complicated solos or for moving a song to a pitch where you can sing along for example.
-It has a variety of inputs, including an XLR in, 1/4" aux in, 3.5mm in and guitar 1/4" in. The normal effects apply only to the guitar input but you can add effects for the microphone input if you wanted some reverb on the vocal for instance.
-If you were to go off and do an acoustic night you could plug the guitar and mic into the looper and send it off to the amp direct and have full control over the volume and tone settings of your sound without having to go through setting up a mixing desk. In fact you could get a loop going and walk about and see how it sounds!
-As a looper you can record loop after loop over the top of each other. You can delete the last loop if you dont like it. My only problem with this pedal is that you can keep undoing loops. You can either undo the last loop or get rid of them all as it appears to combine all the loops to one track. You can change amp models etc between loops so you can do a lead sound over the top of a clean sound or crank up the bass to do a bass line. You can choose whether you want to loop the vocals or the guitar too so you can play and sing but only loop the guitar (or indeed the vocal).
I'm sure it does other stuff but I haven't yet figured them out. After all, reading the manual is admitting defeat isn't it?
PS I don't work for Line 6. I'm just impressed with that this little box can do. As a practice tool or a gadget for solo performance I can't think of anything I'd add aside from my issue with the way loops are stored.
I'm sure those with other loopers will chime in with the advantages of their different systems. I must admit that I didn't take much time to look at the others because I liked the features the JM4 had.