I didn't even know the Xvive U2 existed until a couple weeks ago and when I searched the forum found out I had missed the boat in the deals section for a SDOTD (only post I could find). After spending time researching different units but never buying one (the only one I was considering was the Line 6 G70, but every time that goes on sale it also goes into a back-order status for months), the Xvive U2 suddenly made a lot of sense... its sole purpose is to remove the TS cable from any two things you would use the cable on. No battery pack, no extras, no frills, just a small transmitter and receiver that can be easily plugged into wherever needed. [After reading reviews whining about wireless battery packs, I realized I didn't care to lug one around either.]
 
I impulse-bought one last week and got to test it out this weekend. The only real downsides I have seen is that they do not auto shutdown, and some noted connection issues in dense wi-fi environments. Overall, very slick, cheap, and functional for my uses... simply pop the transmitter or receiver out and swap it around as needed (they are incredibly portable, so can carry them in your pocket too). I can definitely see where a battery pack on a guitar strap would be a PITA (why folks by multiple transmitters).
 
On paper, these have 5 hour run time, 100' range, and 6ms latency. They come with a Y cable for charging both units, and have a firm hinge so the unit can not only be folded, but also positioned as desired and stay put (however, there is no way to tighten this hinge, so I have no idea if over time it will loosen). I have yet to test it for any "recording" situation, but other than that got to run all over the place playing yesterday (I just wanted the sans cable part anyway). The only concern for others is the dense wi-fi aspect... some said no issues, others said they had some... so for folks gigging with these that is something to consider.
 
They also have a U3 (for condenser mics) in the works or possibly released already. They are priced higher, and I didn't really look into them other than to see that they exist.
 
Just and FYI for folks who are tired of instrument cables and do not already own a wireless system.

I have a set.  It's a love/hate relationship. 

I'm a wireless junkie.  I have a relatively new high-end wireless mic (AMT) for my sax that rocks.  Before that I used a Samson Airline clip-on wireless on my sax for about 20 years. 

I also have an Audio Technica unit with a bodypack.  I use that on "keytar" and sometimes guitar.  I attach the pack to the strap so it stays out of the way.  Not a big deal, but the whole setup takes a little time, extra cables, batteries, etc.  So the idea of the little dongles really appealed to me.  

The Xvive U2 system is very cool...when it works.  I've done gigs where it worked just fine.  But in some venues, it's either unreliable or flat out unusable.  In places where there's interference, I wind up hovering right next to my amp (or DI box if I'm on keytar) trying not to lose the signal.  What's the point in being wireless if I can't move?  And the middle of a gig when everyone's waiting to start the song is not the greatest time to discover you're getting some weird interference and have to switch to a cable.  Ugh.

So I gave up.  It's far too unreliable for gigging.  The only exceptions are in clubs where they've been tested and work reliably.  There's one venue I play regularly where they work...but I rarely play guitar or keys on those gigs, so...again, useless.  

I do appreciate having the little buggers around in my studio, though, for practicing or maybe even recording.  Like I said, they're great when they work.  They're simply not suitable for pro use, in my opinion.

Thanks for that feedback Heather. I forgot to mention in the OP that the units only have 4 channels in the 2.4GHz band. I live in the sticks, so the only possible interference would come from my own router; but in a city I can see that as a problem. I got a lot more time with it today, and the unit does carry some weight, so I could also see it coming out in situations with a lot of motion and a loose instrument jack. Flinging the transmitter into a crowd would be rather uncool. Unfortunately, it doesn't come with any type of restraint and has an odd shape that would make it hard to on some guitars. I am not sure I would trust them to gig with either.
 
In a home environment it has been rather liberating for me. Being able to go sit some place comfy without being leashed to the amp was a nice change (and actually gave me a lot more play time).

mettelus
In a home environment it has been rather liberating for me. Being able to go sit some place comfy without being leashed to the amp was a nice change (and actually gave me a lot more play time).

I think they really excel in that environment.  I just leave them plugged into my amp and my guitar at home.  Just like you said, an instrument that's easy to pick up and play gets a lot more love.  They're also handy for jam sessions, rehearsals, etc. where it's not a big deal if they crap out.  It's just a shame they're not more reliable.