As you know I fitted pattern gaskets to one bank and GM to other on the '03 2.6V6 Estate as a comparison -
At last cam covers done today. Gaskets and o-rings were brittle, with 5 of the 6 plug wells full of oil - suprised car was running ok with no misfires!!
Drivers bank fitted with Payen gasket set from allgermanparts and gm o-rings, and passenger bank fitted with a gm gasket set and gm o-rings. As stated before
the Payen set feels as supple and pliable as the gm set, and the fit to the cam cover was same for both types.
Time will tell!! - will update Payen performance vs gm as mileage increases 
Plug wells cleaned out, coil packs cleaned up, and new plugs fitted - still got a smoke screen when started up, but cleared reasonably quickly.
Update - the AGP ones that cost around a fiver have been on the "drivers bank" now for around 16K miles - fitted together with a new set of GM
gaskets (which I had "in stock" and cost me £25 when I bought them) on the "passenger bank". No leakage to date from either bank - in fact just serviced the
car so checked carefully - and in 16K miles only needed to top up engine oil once (after 15K) with around 1/4 pint! Engine and drive both dry.
Both gaskets fitted using new GM o-rings and black sealant - niether bank was torqued, just tightened from middle out in stages by feel (experience). All bolts
checked during recent service for continued tightness & all ok.
HTH
Car has now done approx 25K miles since gasket & o-ring change - no leakage from either bank and all plug wells clean. No oil added since last oil change 8K milews ago. Cam cover bolts checked for tightness - none loosened. 25K to date for a £5 gasket, and still going strong. Seems cost effective to date vs the pricey GM ones. Will continue to monitor whilst still owned.
Mileage now on 127K miles.
Thinking of selling soon, no mechanical issues, bodywork rust free but scruffy - for an '03 CD Estate, not £3K, nor £2K, more realistically nearer sub £1K - reason for sale will be due to purchase of a newer similar sized Estate. Will put in "For Sale" section when replacement located and purchased.