I’m not planning on buying one myself. In fact I’ll probably order the ultralight kit sometime soon here. Just the sheer joy of making my own raft again after making a V3 last year will be worth it.
I now see the post by Apope in his topic on spray decks on his Rapid Rafts, however and these rafts seem to use the exact same system and have been around a little longer. I found some reviews on Reddit and Unchartered’s site.
Apparently the closure is not completely airtight, but when closed properly you can compensate by inflating it by mouth every now and then. The bigger problem seems to be that the rolled part doesn’t absorb shocks very well and therefore is very prone to punctures.