I'll admit to being over dramatic, but I couldn't tell you how many times I've seen a variation of the following on a guitar forum somewhere: "Fender bought Jackson, and then everything went down the toilet." Lots of people on the Jackson / Charvel Forum were afraid of that when the buyout was announced, but if anything the situation improved at least initially. I'm not sure how I feel about that last couple years, but at first things were great with the improved MIJ Jacksons and the return of Charvel.
IMO it's difficult to compare the X series and Soloist line because Soloists were offered in several different series from Custom, to USA Select, Pro Series, MG, etc. For the record I agree with you regarding the X series not being great, but those guitars (DX10D, KVX, & RX10D) were introduced by IMC prior to the Fender buyout. As far as how the various Soloists compare, the USA built ones are the best, and the 1990-1995 MIJ Pro Series are almost identical. The later MIJ Pro Series like the SL-3 and SL-4 were good, but not up to the same standard. These days the Pro Series is made in Indonesia and not that great IMO.