Well I've used the BH-55 for over 3 years now and it's worked well for me. As for the quick release clamp mechanisms; I can stick a camera plate in the clamp, hold it up by that (tripod and all) and shake it extremely hard, it's as ridged and robust as anything, it feels like one whole chunk of metal (not that I advise anyone doing that at all).
I will say that the quick release clamp on my PG-02 had an issue when I got it, in that it wouldn't clamp securely (I believe it was missing a washer/spacer on the shaft that the jaw travels on) but after RRS sent a replacement clamp it's been fine. I trust my Canon 500 F4 II mounted sideways on it all the time if that's any testament
*shrugs*.
In terms of the usability of the BH-55 these would be my thoughts:
Overall it's not quite as smooth and fluid as the Markins head I tried, but I don't find that a problem personally, I don't find it jolting/overshooting at all. I would also say that it's got smoother over time as it's "worn in". Unfortunately I haven't tried an Arca Z1 so I cannot offer a comparison.
If anything I find the slight lack of smoothness only noticeable under lighter loads anyway, with light loads like one of my rangefinders it's a bit harder to find a sweet spot for the drag setting (I found adding a bit of main lock helps), but with my gripped 5DIII + 16-35 it's very pleasant to work with. The BH-55 is definitely designed with heavier loads in mind and with that comes some compromise I guess.
Ergonomically I find the BH-55 is particularly nice, the placement of the drop slots and controls work really well.
On the tripod: as with the sections, deciding on the series depends on what you want to do with it. I find my 3542LS just comfortable for supporting the Canon 500 F4 II (on the RRS PG-02) but anything heavier/bigger and I'd want more rigidity. A 4 series might mean I end up taking the tripod out less when lighter kit is all that's required, whereas with the 3 series it's that bit less of an imposition - but that's just me

. Also I don't particularly fancy the slippery slope of having a tripod for every occasion haha

.