-- Last Updated: Feb-21-11 12:20 PM EST --
I agree with MarkK and Beaver Jack.
I have been there at ice out in early May, the day the Loons arived and it was magical. Been there in late May and it was hideously miserable. Prepare for snow and high winds and you will have anything less covered. How many people show up somewhat depends on weather.
Forgive me if you are not; but if you are novice and/or are inexperienced on lakes with big waves, maybe plan on doing something off the Crooked Lake loop. Crooked lake can be busy and has a few motors, but is beautiful. Portaging a few lakes in will give you a better chance at a campsite and less people. The benefit is the Crooked L loop is more protected than the Clark L loop. Clark Lake (especially the narrows) can be surprisingly nasty if the wind is up. Then if you camp off the Crooked L loop, if you want to do a day paddle and/or complete the circuit of lakes in bad weather, the lakes are small and protected; where Clark L loop has much bigger water.
You will see fewer people during the middle of the week; more people on weekends especially if the weather is nice. But don't be disapointed if you see people; though it is a popular destination Sylvania is special and we are very lucky to have it. If you want real solitude, less traveled rivers are a better choice; pick one that fits your experience level.
Have fun !