Think I'm about 2/3 in now, definitely have a few complaints with it;
-I get a glitch where my map keeps scrolling when I open it up, happens fairly frequently. Gets fixed by dying or resetting checkpoint.
-Some poorly placed checkpoints and they feel too far apart at times. Becomes problematic when you fall off the map since the checkpoints usually occur just before or after combat. There was one part one with a long bridge where I basically lost a good 4 or 5 minutes of progress twice because of it.
-Not only is there no manual saving or way to go back to prior levels to get stuff you missed (in the same playthrough), there's appears to be points of no return near the end of some levels also. Caused me to miss a pretty good rune that I planed on going back for, unless I missed an alternate route.
Otherwise I'm still having a blast, anything else is relatively minor.
you can go back to any level in the mission select, whether you beat the campaign already or not