I have one and a lovely bit of kit it is. Very solid and accurate.
For me, by far the biggest plus is the ease of set up and repeatability that you get. Utilising the movement front to back and side to side together with the built in stops allows perfect replication of mortices in multiple components. Movement in all directions is rock solid with no play whatsoever.
This is the 3rd mortiser I have had and it is the best. It is also the most expensive by far so maybe you would expect that.
It could be argued that it is perhaps, OTT for most home workshop setups, but if you like nice stuff, have the floorspace and are willing to pay that bit extra, this is a really nice machine that, together with decent chisels, won't disappoint.
I have the Powermatic version of the same. Great tool. fwiw, this version of the Jet in the USA now has a tilting table.
If you have the cash and you build anything using tapers that you cannot easily cut mortices prior to tapering, or if angled mortices are used frequently, I would consider getting one with a tilting column (like the General) or the tilting table of the Powermatic 720.
As mine doesn't tilt, I cut tapered wdges the length of the bed to compensate, which works. The tilting mechanism just saves setup time.
But I would buy this one again. Solid, fast, repeatable.