Yes i have used the super concentrates by wonder flavours and to be fair the % ratings are tosh you need 5% or more to even taste most of them. i mixed rice pudding, shamrock shake and mixed berry as well as sweet and tart all steeped for minimum 4 weeks all tasted weak at 3%.
try them at 5% to start with then higher or lower till you feel they are good. by the way i mixed 80vg20pg
I've just mixed the Bubblegum at 2.5% and it's not too bad. I could see many people being happy with this and I've bought premium liquids that were weaker.
These first 2 are a bit rough though. If I didn't know better I'd be blaming my nicotine - hopefully they'll get smoother with time.