Update: been reading Line 6 manuals and forum posts, and found some interesting stuff.
1. The new G55 has a bunch of useful upgrades- a new RF mode that's less likely to impact, or be impacted by, other wi-fi devices in the area, BNC connectors for use with external antennae, just like the big-boy G90, etc.
EDIT: Forgot to mention that the G55 transmitter is confirmed compatible with up to 2800mAh rechargeable cells. They still tell you to try before you buy though, as rechargeable sizes vary so much from published specs, not just for diameter, but length as well. Screws up the battery meter calibration, but oh well...
2. The G55's new RF2 mode is incompatible with the older gear. There's a v2.0 firmware upgrade for the old G30's and G50's that makes them compatible with the G55, but it requires access to a specific model of Line 6 receiver, or a trip back to the factory. Alternatively, the G55 can be dumbed down to the old RF1 mode, so that they can play better together.
3. They've also come clean about causes of interference and dropouts and ways of avoiding them, instead of simply claiming that their gear is bulletproof. The manual is much longer, and includes advice about issues like maintaining a minimum 6' distance between receiver and transmitter, a suggestion to get the receiver up above head height, the admission that only 6 to 8 out of twelve channels are likely to be usable in the presence of other wifi systems, etc.
I expect that this would be a good read for users of older Line 6 devices, like the G30 and G50.
4. As an old Line 6 user, I was amused to see that their customer support is as slow as ever. They first started promising an advanced user guide for the Relay systems in 2010 or earlier. It's now 2013, and it's still not available. Looks like the G55 manual is the long-promised advanced guide, but who knows how much of the content is/isn't applicable to the old gear?
5. The flimsy plastic internals in the TA-4 jacks on the older G50's have been replaced by metal ones, but it's not clear when they made the change. There's a warranty replacement program as well. Of course, this doesn't help if yours is out of warranty, and Line 6 repair service is backed up again.
The 6-foot limitation could be an issue on crowded stages, as could the tendency of the old G50 to interfere with Bluetooth and other wi-fi devices.
This could be an advantage for the new Sony system with its 1-foot minimum receiver/transmitter distance, and low-interference RF2 mode. Of course, that could also just be marketing smoke and mirrors...
