Did the Allagash in '82, so my comments may not reflect current conditions.  First of all, I did enjoy the trip and do recommend it, it is a beautiful area.  We did it as a two week trip, (early in June) so as to have plenty of time for fishing, and even went up the Allagash Stream to Allagash Lake and spent a couple days there as a side trip.  You could easily do the route (w/o the sidetrip) in a week, and probably could do it in 5 days, and possibly less.

Allagash Wilderness Waterway - Wilderness? not hardly.  Camping is similar to BWCA.  Camp only in designated campsites, which include a fire pit, outhouse, and even a picnic table, and a log frame to strech your tarp over the table.   The "wilderness" waterway is a narrow corridor (i think a mile on either side of the river?)  through the Maine North Woods, which is basically all timber company owned.  There are two or three roads that cross the route.   But, it is remote, and on the river part, we saw moose every day - probably saw 30 to 40 moose for the entire trip.  I do not recall it being crowded when we did it, though Alagash Lake (which has fly in access to a nearby pond) was busy.  We probably saw one or two other parties a day at most, and I think there were some days we did not see anyone.  I think we were able to get the campsites we wanted every night except one (or maybe two?) day on the river, where our first choice was taken

There are two parts to the trip, about 50/50 lake and river, depending on where you put in.   The lakes are mostly big water, and interconnected.  I can remember one portage between two of the lakes, maybe a 1/4 mile, and not a big deal.  The river section starts at a dam at the end of the lakes, and it is about 50 miles if I remember right.  There are several sections of rapids.  The Chase Rapids is nine miles long.  There is a ranger living near the dam.  Every morning, he opens the floodgates and raises the river level so you can run the chase rapids with high water.  For a modest fee, he will portage your gear to a point somewhere below the hardest
part of the rapids.  He will also portage you and your canoe if you want.  So you have 3 choices - avoid, run empty, or run full - we ran it empty.  You could even do it a couple of times if you shuttle twice.  I would say that Chase rapids are class II - its lots of rock garden, mosthly just dodging rocks.  I don't remember there being any drops, or big standing waves.  We did it in a 15' Grumman, and we did hang up on one rock - otherwise, I think we ran it clean.  It does take some white water skills, but it was not desperate.  The other rapids were no big deal, and I do not remember there being any portages around them.  They were also just rock gardens.   Alagash Falls is a mandatory portage, not too difficult and not too long (I think).  We camped there a couple of nights - had great fishing right below the falls for Kokanee Salmon.  Other fishing was pretty good also.  Caught a 20" heavy Brookie in Allagash Lake, another 20"er, and a 20" Lake Trout in some of the other lakes.  We camped near the dam one night, and spent the next day hiking and fishing the Chase Rapids (we waded), and caught bunches of 8 to 12 inch brookies.    The last day on the river, we had to get out and walk the canoe through lots of gravel bars - it gets wide and shallow on the lower section, and we did see several canoes with motors comeing upstream.  They weld a pitchfork on the motor to protect the prop, and drive right over the shallow gravel bars.  You will see more people there.  We pulled out on private property, and paid a fee, at the bridge in the town of Alalgash.  We hired an outfitter to drive our vehicle to the town, where we picked it up.  If you do the trip earlier in the spring, you can continue the Alagash to the St Johns River, and do another 50 or 100 miles on that.  

That is all off the top of my head, which is sometimes pointy, sometimes slanted, and sometimes even right.  If you'd like, I can look through my trip album and come up with actual details, campsites used, mileage, address to write to to get a map, etc.  It is black fly country, of course.

How "crowded" it is, I think just depends on when you do the trip.  I would recommend not starting on a Saturday, but try for Tues or Wed.  If you start at the same time as several other parties, I can see where you could all be traveling at the same pace, down the same route, and it could seem to be crowded, and you would have competition for campsites.  You do need to reserve an entry date permit, and pay a fee.  I would not want to do the river section in a straight tracking kevlar lake canoe, but you do not need a white water canoe either.

Louisa, you can probably find more current info on the Northeast Paddlers Message Board, or just do a web search for Allagash Wilderness Waterway.  I did that several months ago, and found several websites with info, outfitters, etc.

I would definitely do the Allagash again.