I had bolt-on valve covers for my VW (aluminum and finned). For several years (like, 10 or so). I liked the look and mine never leaked so long as I used new O-rings on the studs every time I put the covers back on (O-ring then hold down nut) and cork valve cover gaskets. But it gets to be a real pain when removing the cover takes longer than the actual valve adjustment!
I finally got tired of the wrenching and went back to stock valve covers and bails. Yeah, not as purdy lookin' but much quicker and easier valve adjustments now