The concepts are obviously quite similar. javelin looks really cool actually - it supports dosync updates of cells. I have read through the docs for javelin and hoplon a little, but honestly haven't gone too deep. It seems that because all the rage in the cljs world is around React, maybe hoplon/javelin hasn't gotten its due attention... I can tell from looking at the source for javelin that its implementation is quite different from freactive's and is more based on macros. Maybe some from their team can comment on the pros/cons.
I'm pretty sure javelin should work with freactive out of the box if you want to give it a try.