The summer solstice list is up (thanks @PlayForFun):
Time for a count, just for fun 
4 stars: I have pulled 30 of the 93 in the portal
3 stars: I have pulled 45 of the 83 in the portal
(This is according to my hero museum at least - I may have fed some others away before the museum came online - yes, I did! Morris 
)
I guess this is about the best odds for something new in the game. Of course not all new things are very good, but I will go for one 10 pull. In Black Friday last year, I had a really great 10 pull and got a bunch of good 4s and 3s, and one 5 star: Hel. I use many of them constantly - some have never been leveled and probably never will be (Valeria, for example).
I know a 10 pull can also give me +10 to the FS counter and nothing else! If that happens, I have to figure out if I will try one more 10 pull. I can do it, but just barely - I only have enough gems for two 10 pulls and one roster expansion. I am down to 8 roster slots now, as I decided not to go for the SE last time - now, I am up to 15 for the next SE.
The primary other place that interests me for a 10 pull is CF1 - but only after all of the useful 4 star costumes have been released, and that is likely to take months. (I said the same thing about the costume chamber, and then ended up not waiting for all of the second costumes - I did two 10 pulls when they counted the costumes towards FS.)
Well, I have a few days to figure it out!