I've shaped two Serb willow bats, used one most of last season. Performance and density aren't any different from normal willow, but there are a lot more low density clefts available as it's a big untapped supply. The only unusual thing I was told by my supplier is that unlike with English willow the moisture content doesn't appear to correspond with cleft weight as normal, you can have some very dry heavy ones. Imagine that's more of a processing thing than an inherent property though.
In the particular case of my bats, one is massive and one on the small side. Massive one is only a G3 but is a proper gun, the other one is a really nice grainy G1, feels great in nets but haven't used it outside yet. The sound they both make is fantastic too.