We had charters run by them when I first started at my small station in Wisconsin going to Wendover, UT (ENV). I don't know how long they went with Xtra Airways prior, but it wasn't long after I started until they switched to Allegiant. Lots of pretty large delays.

Edit: and as far as competition with SY, they're pretty well-defined as catering to the MSP and now LAN markets. Don't think people will be going from Caljet's markets to Sun Country's or vice versa over price.