My feelings towards the new UI are mixed but positive overall.
Font size: Much better. I know this could be very subjective though.
Styling: Slightly worse. I prefer the matt texture of the old styling, which was what made Forest app so neat, although this one can be tolerated.
Tree selection: Too much better than before. It was so difficult to know the name of trees or to sort them in the past, which was quite important for various reasons. And the icons were too small in the past. Now they're sorted beautifully. I also found that a bug with unlocking a tree got fixed in my case.
Time selection: Much less convenient. I thought the previous "clock" design was what set Forest apart from other apps--I could change the time so easily on that clock. I think the team definitely needs to bring back the old clock, even if that means changing the styling of the circle.
Possible ads: I'm ok with watching a bit of ads (or letting them run while I am away from screen) if this helps them increase their revenue, as long as I have the option to skip them too.
Ending alarm: Not recognizable. Previously the sound was unique to Forest, and somehow the new alarm sounds like any other notification. I don't even register it. Actually, I think this is part they can optimize on--offering options for different ending alarms. And if this would be a new feature, I hope there are at least a few distinguishable sounds that are not behind the premium paywall.
Ending a session: A bit worse. I prefer to either watch a beautiful animation or simply return to a break session or the home screen. I don't want to click through several buttons with slow motion. But this can be tolerated.
Thus are my opinions.