No loops for me, I'm a big fan of the Albright knot, instead of the nail knot. Nail knot is ok for trout fishing, but ever since i lost a nice a$$ steelhead to a nail knot pulling off the tip i have used albright and haven't blinked since. As far as knots getting caught in the guides, i guess most of the time i'm using sinktips with leaders no longer than 5 feet, so if i'm fighting a fish, the knot dosen't even go through the guides.
On loop connections- I don't think they are the best for connecting the leader to the tip in the type of fishing i do for salmon and steelhead. Especially with a spey rod, I want the line to be able to easily turn over my whole leader and fly. With loops that aren't PROPERLY constructed, especially when connecting the leader to the tip, let alone loop connections to cheaters from the tip and to the belly,etc.; if there is not a flawless transfer of energy, a "hinge" effect is created which inhibits turning over a fly and leader. The albright knot creates a great transfer of energy if heavy enough leader is used, i usually use 30lb. maxima, and has never once slipped off my tip, unlike the nailknot which especially tends to slip off T-14.