I've tried few of them and many suffer from being too front heavy. ( your arms get tired supporting the weight of the full rig or you have to put up with a front support rod that sticks out from your belt to support the weight )
The only one that I've really liked has been the Zacuto rig with an offset counter-balance weight, so none of the weight is supported by your arms, and it does not require an intrusive support rod jutting out from your belt.
The Zacuto rig is super pricey, but I think I am finally going to take the plunge after lusting for this gear for the past year. ( I plan on using it with other cameras, so it should last me for quite a long time )
I use Red Rock Micro's. $400, but comfortable and well built. My only complaint is that it doesn't have the counterbalance like the Zacuto, but I'm not willing to pay that much. Plus, I've managed to adjust it just right that the weight isn't that big a deal.
I've only got one so far...and it works great on a naked HPX-170. Will probably get one more for the M2/M3 + Mattebox add-on in case I ever go shoulder-mounted with that rig...I'm thinking it'll only need 2lbs of counterbalanced weight, but it might possibly need 3lbs. We'll see.