Great fun, with loads of features. It is quickly becoming a must have in my gear set.
The Good: 2 separate effects loops, electric guitar oriented with the knobs on the top, mic/acoustic guitar oriented in the menu system. Aux input is selectable as to which effects to use. Cd/mp3 input. Level and/or volume controls for everything, combined with a direct outs let this thing work as a 4 channel mixer if needed. The loops and drums are great, and you can add more as wav files on an SD card. Firmware can be updated, which has already improved my JM4 (came with V2.0). The looping onto a drum track is easy, and overdubs are more easy. Record your own jams/songs, and transfer to the 'puter using an SD card for further editing/ emailing/ cd making. You can bypass amp modeling, and just use the effects, or you can skip those also and run your POD into the JM4.
The Bad: If you are going to rely on it for background during live performances, you may want to tailor your sets to give you time to update the memory during the set rather then planning a break every 1/2 hour, it is pretty quick if your organized. It doesn't have an input for my FVB Short board. The 2 second preview of songs & drums that are internal, does not preview what is on the SD card as you scroll through the listings.