We came to Sly Park for my sister's birthday in mid-July and had a fantastic time.  The water level in Jenkinson Lake is low, as to be expected, but there's still enough for swimming, kayaking, boating, and fishing.  

   There are different campgrounds, which seem to be designed to meet different needs:  at Pine Cone, for example, you're right on the water (great for families with kids) but the campsites are cheek-by-jowl with everyone else; if it's privacy you're looking for, reserve a site in one of the places farther down the road, such as Hilltop, Kamloop, or Dogwood.  For the most part, these sites don't have water views but they're quiet and private--and the water is just a short walk or drive away.  We stayed in Hilltop, which has a loop and a spur, and we had the entire spur area to ourselves--it was fantastic.  (The park didn't have a lot of campers--maybe the summer heat?)  Our site was in the shade of the conifers (it smelled great!), it was about as quiet as it could be in a campground, and with no light pollution nearby, we could see a million stars overhead at night.  Just perfect!  

   The only reason I gave it 4 stars instead of 5 is because Sly Park only has vault toilets and spigots for drinking water but no other accommodations like sinks or showers.  When you're staying for more than a night or two, a shower or even just a sink would be really nice.  Cell service (we have Verizon and AT&T) was only available by the entrance kiosk.

   Despite the lack of sinks/showers, we loved Sly Park and are already planning a return visit in the Fall when it's not so hot.