In reply to andy farnell:
I've just gone from wearing the Stix to the Booster S so hope this helps!
The Stix for me were a lot wider (although you'd expect them to be as they are a slipper and not a velcro shoe!) but I have really narrow feet anyway so I don't normally go for slippers if I can help it. I found I needed to downsize a bit for the Stix as my normal size was just too baggy in the upper, even though they fitted comfortably in length.
For the Booster S I was back to my usual climbing shoe size, I was lucky as I had worn the Boostics which are a very similar shoe so could estimate well from those. The last is different, the Booster S seem a lot narrower at the heel than the Stix and are generally more downturned and 'pointy' in the toe. The precision and fit are great - similar to the Boostic but you can get even more "pull" from the downturn.
I'd say the heel of the Booster S is very similar to the Vapour Velcros if you've ever worn those?