What’s the word on this handle? How does it compare to other handles? Does it mount on most saddles without adding drilling/holes?
I’ve not used one but I looked into them when upgrading my 36. For a long time they were the only off-the-peg touring handle available.
They look like a fairly neat design, they’re compact and presumably add little weight or bulk to your uni. There’s also the option of a nifty cyclometer mount.
There’s the rub. The GB4 bolts onto the underside of the front of the handle.
I’m not sure if you can use the existing holes in your base or if you’d need to drill new / additional ones. Also, most saddles aren’t recommended as being strong enough to take the load involved. For Miyata style bases I believe you can purchase a stiffener plate which will allow the saddle to support the GB4. For KH style bases, you’d need to upgrade to a carbon fibre base or rig up some kind of reinforcement yourself (ISTR pdc was able to support a Reeder handle by reinforcing it with the brake mounting tube of his KH rail adaptor.
I’ve seen folks who have successfully mounted the Reeder onto viscount seats (which have a steel base) so maybe something like that would work.
In the end I waited and bought a T7 handle, which has a built-in rail adaptor and works with unmodified seats. It’s bulkier than a GB4, and I’d be surprised if it wasn’t a fair bit heavier. The bars themselves extend further forwards than the GB4, and higher - some people seem to think this is a good thing. Having never used the GB4 I can only say that I find the T7 to be comfortably positioned. The T7 also includes plenty of space for mounting lights, cyclometer, bottle cages, etc. There’s a handle at the back too - this can be useful and may serve to balance the extra weight at the front a bit - although you could cut that off if you didn’t like it.
I liked mine so much on my 36" that I bought another for my 29. 
My only gripes are: 1) some people (me included) have had problems with the bottle cage bosses coming off. Roger at UDC UK was able to fix this for me free of charge. 2) there are some sharp corners on the front handle which hurt if they hit you and also get chipped easily when dropped. I really like the T7 as a place to mount stuff and as a way to take weight off the saddle whilst riding long distances. I wouldn’t want a distance uni without one 
I’ve heard rumours a newer version of the T7 is coming out without the sharp corners which would be even more awesome - but the current product is good.