Stacking helps a lot when mass tabling but I think it's a little detrimental to your play. Now and again you will lose track of the action, it's difficult to use your timebank, now and again you'll accidentally fold aces preflop, and making notes is difficult when you need to pull a table aside to view results.
Stacking never worked for me, for those reasons and more. I have 3 1920x1200 monitors and can tile 9 per, for a max of 27 if I really wanted to punish myself, lol. Tiling for me is just way better than stacking.
The most I've ever played is 16 and when I initially tiled across all 3 monitors, the huge distance between the monitors caused me a neck ache from the large and constant head movement required, and I also often missed action that was occurring outside my peripheral vision when focused on one table. So I scaled down the tables to 9 per monitor which allows me to comfortably tile across 2 monitors, leaving the 3rd monitor for the poker lobby, tracker, browsers, music player, etc.
Tiling allows me to have a more peripheral awareness of multiple tables at a time -- even though I'm not consciously focusing on multiple tables at the same instant, I am still pseudo-aware of what's happening on multiple tables when they fit within my peripheral vision. Stacking hides everything but the focused table, which I was just never comfortable with. I know some players compromise and tile+stack, but since I have enough monitor real estate to tile all the tables I expect I'd ever play, I don't need that compromise either.