Curl Keeper is the one product I consider my HG. I have tried it an assortment of ways, and though I definitely prefer it with a leave-in and some gel, I have never had a day where I was disappointed in my hair no matter what format I used it in.
Like I said, my preference is over a leave-in with some gel. I've used it over Yes to Cucumbers Leave-In and over Giovanni Direct Leave-In. I've used it with AG:Recoil, LA Looks Sport Gel, IC Hair Polisher Styling Gel with Sparkle Lites, and Herbal Essences Totally Twisted Scrunching Gel. All combos worked well.
I've also used CK over a leave-in only, with no gel. I still love the feel of my hair, and it styles well. But there is very little hold, so my hair loses control faster. In other words, 2nd or 3rd day hair won't be as easy/nice.
When I ran out of leave-in, I used CK with a gel only. I still liked it, but my hair was drier without the leave-in.
A few times I literally used only Curl Keeper. You do need more product if that's the route you go. But it makes for some seriously light curls. The product is very watery, so it doesn't make your hair feel like there is product at all when you just have CK. Again, there is minimal hold so if you go outside in the wind, or humidity, the results may not be to your liking.
The product itself is awesome. I do hear it has almost identical ingredients to personal lubrication. KY is expensive as well, but at least it's in drugstores. But several people have mentioned that a generic brand of lube works fine. I may try it some day, but I'm not someone with personal experience in that department
I highly recommend the product. It lasts awhile and is worth every penny. Try a small bottle first, though, to make sure you find it is worth the purchase. I buy the massive bottle whenever I have the money, though.
NOTE: I am currently milking an overnight "plopping" and I have Kinky Curly Knot Today, Kinky Curly Curling Custard and Curl Keeper in my hair. I'll be pixie curl diffusing in the morning, so I can offer feedback with that combo tomorrow :-)