I have had many batches of XSF, in fact I was possibly the first person on English language table tennis forums to strongly advocate for their use (against considerable skepticism). Some recent batches if XSF balls have had some complaints about roundness. This was not a problem with XSF balls made in the first half of 2015. However, I have not heard a complaint about any other brand of seamless ball. My impression is that there is a QC issue in which XSF has accepted some substandard balls lately (late 2015 and early 2016) and decided to sell them as 3-star. Greed. Shame on them.
However, I have not heard or experienced anything like that from one of other brands of seamless balls.
So if you are in doubt, avoid XSF and simply buy one of the many other versions of seamless balls, like Xiom, Nexy, Kingnik, Joola, Yinhe, etc. etc. There are now many brands to choose from. I suspect each company can set their own QC specifications with the factory that makes them. For a variety of reasons too obscure to go into here, I have some reason to believe that there now may be two different factories making the seamless balls.
Seamless does not bounce or fly through the air exactly like celluloid or like Nittaku Premium. There is no question about that. (Actually it bounces a touch higher and seems to me to play a bit slower). But it wears substantially longer (on average) and is quite a bit cheaper.
Personally, I prefer playing with the Nittaku balls BUT if price is an issue, the question then becomes what is the best overall price/quality ratio. If you look at that, and consider average durability as important, then good seamless balls win hands down. Bear in mind, you have to buy the three-star versions. The "training ball" versions are awful.
I use XSF balls for training out of a bucket. I play matches and free play with the Nittaku.