Having recently got access myself (Thanks for notifying me of that Sitrec! 
), I can say that I’m thoroughly impressed so far. There’s not huge amount of pre-made trees, but what you can do with the available sample assets is rather fantastic. The ability to wrap vines and the like around an imported mesh? Awesome!
Assuming you can model, you should be able to import additional branches, trunks and so forth. That, or you could wait and purchase additional assets from the store when they become available. I’m very interested to see what pricing structure they come up with for them.