Bag-A-Bug traps are very satisfying when you see how many bugs you kill, but they don't do much to save your plants from the little bastards.
I've found Sevin to be almost 100% effective, as long as I respray after every rainfall. The only thing I gave up on was roses. The beetles burrow into rosebuds as they open, and you can't apply bug poison quickly enough to prevent that.
To me, milky spore is a total waste of money and time. It takes forever to propagate through your lawn, and even if you kill your grubs, millions of bugs are flying around from other lawns.
I don't have much of a Japanese beetle problem now. The deer eat most everything before the beetles come out.