I've made two transits, however they were both around ten years ago, so I don't know what might have changed. I never considered using an agent (although I would have been completely open to it if I'd had any inkling that going without one was unduly hard). Both times I found a good taxi driver to be the best help, and super economical, too. They know where the various offices are and can make a day of taking you around to them in the proper order. The admeasurer will come out to the boat (although if your boat has been through before you can use the same number and won't need to be measured).

More info that you may not need:

Taxi drivers can also help you get (and dispose of!) garbage-bag wrapped tires for fendering (although the best way is to catch a boat as they come into the anchorage fresh from a transit and offer to relieve them of theirs), and they have connections to lines for rent (you need something like four 125' lines of a pretty hefty diameter; I didn't have those, but even if I had I might have chosen to let rental lines take the abuse).

For going through, I found a small pamphlet type book helpful, but I can't remember the name. Probably the Internet will substitute handily these days.

Also, you can crew through on another boat prior to taking your own boat through - this can be helpful to learn the ropes. Each boat needs four line-handlers plus the skipper, so plenty of people will usually be looking for help. On the second transit we had an extra person, which was nice, as you need to provide refreshments for the crew/advisor/understudy, and then too you have a spare person to take photos.

When I went, most boats made it through in one day; not sure if that is still the case - I think maybe more people stay overnight in Lake Gatun now, but I'm not sure. We also could specify which "tie" methods we would or wouldn't use (center-lock; center-lock rafted to other boats; side-tie; side-tied-to-a-tug). We saw a few boats that had sustained serious damage using the center-tie-rafted method (think of one boat's cleats holding three boats...), so we said we would only use one of the other three methods. We ended up with a combination of center-lock and side-tied-to-a-tug.

My info may be out of date. and.... now the Internet provides