Hi
I've been using the libra hack about since february. To be fair it is the only treeless I've tried (so nothing to compare it to) and I chose it after doing much research, but I have been very pleased with it. It is very comfy and my pony goes well in it.
We had tried various treed saddles before and I even had a saddle fitter out but wasn't impressed with the options available, they either needed cruppers and anti slip pads or they would move forward or slip.
I did push the boat out and buy the christ lammfelle pad which is pure luxury to go underneath it, and it has never slipped round or forward. I bought it in brown, but like it so much I would eventually like a black one!!
The only thing against it: I got a synthetic one and if you catch it slightly, the material comes off, so if you can afford, go for leather. I only hack so for me I don't have any complaints about it.
Good luck in your search.