I have a GR-55 (for 2 yrs) and i find it totally awesum, the mentioned latency is only on the midi/PCM synth side of things, the modeling part is latency free, plus the multi effects.
With the gR-55 you get two PCM synths (910 tones each), the modeling (guitars, basses, acoustics, etc), plus your normal pickup. When these four tone sources are all mixed into a composite tone, the synth latency is barely noticeable.
I would not recommend the Roland G-77B, that is 1984 technology, things have come aloooong way since then.
Free Boss GT range and GR55 editors available at fxfloorboard.sourceforge.net