I've had good luck with their queens in the past, and another beekeeper in the village has a very strong hive headed by a Koehnen Carniolan introduced into her hive last year. Here's a photo of it at the beginning of April.... right in the middle under the bees is a pollen patty I had given her... the bees all over it. I feed within an empty chamber.
Last year I had a bottom mounted Sundance Pollen trap on a hive and gathered 5 or 6 quarts of pollen, which I froze and used in my pollen patty mix this spring, and gave all the hives a series of pollen patties beginning around the first week of March. It really helps kick start the brood rearing, especially for those clusters that don't happen to be parked near any frames with pollen. It's also good to give them sugar syrup, 1:1 ration of sugar to water.
The pollen patty recipe I use is ... equal parts of feed grade Brewer's Yeast and Mann Lake "Bee Pro", ...then add to that between 15-20% bee pollen by weight, 1 tablespoon olive oil, add some honey from my bees, and enough 1:1 sugar syrup mixed in until the consistency is like peanut butter. I paddle out fistful portions and squish between wax paper, ... here's a patty ...notice the bee pollen pellets within.
This supplemental spring feeding will help your bees build up for the early flows such as Black Locust which have that light colored wonderful, elegant honey which doesn't crystallize quickly.
Right now the Red Maples and Willow should be giving the bees some good pollen.
I hope your bees are all doing well in the warm spring weather.
Also, here's a great beekeeping forum that I've found to be a great resource... http://www.beesource.com/forums/index.php