In my area of the country, depths up to 9" were a somewhat regular thing with the 5900 Di Pro. (Targets deeper are rare around here too so... ) Would I run it over a newer machine starting green? No way. Running with a V3 now and an MXT would be a must have ,for me still ,as a second machine. I have never owned one but have had the chance to use them quite a few times now... and at the end of the day....always have had as much stuff as anyone. Fast, Fast, Fast response, simple and plenty of stable punch. Loving gold is THE major added bonus.
The main reason the old timers like the 5900 so well is the manual ground balance and the ability to just glance at the analog knobs to adjust on the fly. Like Elton said, the ones who loved 'em, really know how to use 'em. I used mine as a second machine (as I had most all coils for it) and used it as a specialty detector. A Bigfoot and the 5900 was really something on sports fields and used a 5.3 in overgrown areas. Eventually acquired the same coils for my DFX and sold most of the 5900 setup. Had many moments of sellers remorse (because of the good memories) but taking it over an MXT, given the choice...not a chance. Can't justify the outlay for another machine but someday...