The GR-55 allows processing the guitar signal through multiple effects, including COSM, as well as converting the 13-pin signal from individual pickups to MIDI, which can be used to play two on-board PCM synth sounds and/or sent out to other synths (including computer via USB). This makes it extremely versatile both as a multi-effect and guitar synth.
The on-board PCM synth sounds are familiar Roland fare, with the presets being not particularly impressive on the average, but the sounds are very tweakable, and combining them with effects and other sounds provides endless combinations. An unfortunate thing is that the on-board synth cannot be played by external MIDI, so you don't get a bonus Roland sound module for other use. The guitar to MIDI conversion works reasonably well, although with some patches it takes careful playing to avoid audible false notes sometimes. Careful set-up of string levels is necessary, and online forums are a better source of guidance for this than the product manual.
Physically the device is very robust, and the foot controls, including a dual-function expression pedal, feel great. Navigating the menus is reasonably fast with the dial and buttons, especially if already familiar with Roland synths.
Computer USB interface is not class compliant and thus needs custom driver, which can be a problem in the future since Roland is not very active in supporting older products. But fortunately the GR-55 has MIDI DIN outputs so this functionality is not dependant on USB.