I'm a heavy user of of both synapse (orbweaver + naga) and LGS (performance mouse mx, anywhere mouse MX, g9x, g700s) and i have to say....they're both fairly identical.
LGS still wins with mice that have onboard memory, but that's about it. much of the synapse hate was for the online-only factor and they've must have cleaned up synapse 2 ALOT because I've only had one issue with it - it's very bad at keepingtrack of synced profiles and asks me to redownload the latest profile even when that profile came from the current device.
haven't had the g600 or the nagas, but everyone complained that all MMO mice had membrany feeling buttons and it was hard to tell your position on the 'grid' by feel alone. both problems have been solved - positioning is VERY accurate.
That's the brilliant part about the G600, the keypad is shaped so that you always know which button is where. When I got mine there was no learning curve at all.
Also, the reason LGS wins over Synapse is that when it detects a game(unlike Synapse, LGS will auto-detect many games) it comes pre-loaded with a list of commands for that game that you can then drag and drop onto the buttons you want them on. When you then exit the application or tab out, it also deactivates the profile and reverts to the default as long as there is no profile assigned to whatever takes focus next.
All in all the LGS is much more convenient than the Synapse. Also, the macro functionality in LGS is better than in Synapse. For instance, you cannot make a latched key in Synapse no matter how hard you try, which is a pain because many MMO games benefit greatly from latched keys, such as making a key to toggle right click on and off for a look toggle. That's something you can't actually do in Synapse.