Another vote for QLX-D.
Had an old X2 digital system (900MHZ) that was a dream. Aside from my transmitters getting old and the connectors getting oxidized occasionally, it worked beautifully. No dropouts. I sold it cause I wanted 2 wireless units in 1U of rackspace, so I got a pair of Line 6 G55.
Never had problems at rehearsals, or even at shows during soundchecks, but as soon as the room filled up, I'd have dropouts like no other. I also work as a FOH engineer / RF tech for a touring regional act. Several of the musicians were using GLX-D and/or Line 6 wireless. Over the course of the last couple of years, dropouts became more and more frequent.
It's my opinion that 2.4GHz is no longer a viable band for professional / high availability wireless systems. There are simply too many consumer devices and technologies that operate in that band. It's too risky, especially in a touring environment where the RF landscape can be drastically different every night.
I bought a Shure QLX-D and it's like I have my old X2 back, but it's even better. Range is insane, tone is basically indistinguishable from a cable, and the RF side is professionally done. Wide tuning bandwidth, good filtering on the receivers, and can easily be coordinated with other UHF/VHF systems using something like Shure Wireless Workbench. I convinced the band to upgrade to QLX also, and we haven't had dropouts since. For price to performance, QLX is the way to go. If you need even better RF performance, ULX-D is the next step up. QLX-D transmitters are partially backwards compatible with ULX-D receivers if you ever decide to upgrade down the road.
I also have a Shure SLX system that I tested out for guitar for a while. The SLX is analog and suffers from some high end signal loss and uses a compander. The RF performance is also much worse than the QLX unit. I generally wouldn't recommend the SLX, but it's in the same price range as the 2.4GHz stuff (especially on the used market) and will likely perform better if you're experiencing a lot of dropouts using 2.4GHz systems without a HUGE down-grade in audio quality.