STN's been under-served for quite a number of years, especially with regards to holiday destinations. The large presence of another airline I think has been long overdue and Jet2 have proven to be a great fit.
With regards to their competition I think it's quite broad... arguably FR, EZY, MT and TOM at STN but also will certainly be attracting passengers from LGW, LTN and SEN to some degree perhaps from various carriers.
I must say though for EZY their backward movement at STN appears to be back-firing slightly. Would not have been nearly as lucrative for Jet2 had EZY not moved so many routes and aircraft away.