ProPress fittings
Some interesting questions on seal, strength, and longevity. As someone who has worked with this system (and the new ProPress Stainless) a great deal, i can make a few comments.
You can turn a fitting 1/2-2" and re-press it. The re-press is only needed to insure the pullout strength is not joepardized. The seal will remain intact even if you don't re-press it. With Viega, there is a grip ring in larger sizes that will not allow you to make changes once pressed (2 1/2, 3, and 4").
You do have to plan ahead in some areas (like stacking or bathroom groups) as you must be able to get the ring or jaw around the fitting. However, with the rings (both XL and mini) you can press right up to something like a floor penetration without issue.
The pullout strength has changed a bit with the newer copper fittings, but I can say that several years ago we swung a 14 story riser (4") from a roof top crane at Scottsdale Waterfront. I am pretty sure that was well over 500lbs on the top fittings. No issues! Those were the old style bronze fittings though.
The metal to metal "seal" is mainly a static seal for the "o" ring. After the press the "o" ring is no longer in the medium or path, so the EPDM does not break down like it does in fixtures. Anyway, they (Viega) have a 50 year warranty.
It is more expensive but you can realize well over 25% labor savings.
If you can get in with the reps., try and tour the factories. You'll be sold even if you have never pulled the trigger.
As with many products, propress has taken on a generic term for press technology. Truth is they are not the same. ProPress is a trademark of Viega, and there are several other manufactures out there (which are not of the same quality or construction). I only make that statement because I have seen and heard of issues with "propress" that turn out to be some other press.