That is a beautiful area of the BWCA. Of course i'm biased because I've been to only 3 areas of the BWCA. But of the 5 times I've been in the BWCA, 3 of them have been in the Caribou Lake area. That says a lot. 1 trip was with an aunt, another with an outdoors club and the last with my wife. We pushed ourselves more with the outdoors club and did a loop with the 1 mile portage from Meeds lake to Poplar on the way out. Gasp! But the other trips were out and back trips where we set up a base camp and fished.
The portages are very flat and well traveled. Not difficult at all. As far as crowding, May is the least busiest month of the year according to my BWCA guide followed by Semptember. So shouldn't be an issue. Dress warm!
Keep an eye out for moose, we witnessed a bull moose checking out a cow and her calf at the portage from Carribou to Meeds Lake. It was quite amazing.
I'd recommend the campsite on the west shore of Carribou, there's actually two, one on each of two tiny peninsulas and the northern campsite/peninsula is very nice.
There is a very nice put in on the north-western point of Poplar Lake with a parking lot and bathroom.
Have fun!
Bill