So let me present the experiences of someone who has owned and lived with a TS Selespeed SW for 11 years; I bought it new in 2002. Apart from the standard maintenance costs, cam belt, variator, etc there have been a couple of significant issues;
1. De-selecting 5th gear - when changing up the box into 5th, it would try to engage then drop into neutral (interesting when accelerating up to motorway speeds). I researched this and other forums and assumed Selespeed fault, but nothing showed on AlfaDiag; swapped position sensors anyway and adjusted clutch push rod, no change; took end plate off gearbox to check selector fork and synchro hub to find the nut on the main-shaft was loose and the sunchro sleeve was chewed up. Replaced sleeve and nut, torqued up properly, problem fixed. It wasn't a Selespeed fault, it was a mechanical gearbox fault that could just as easily have affected a manual box.
2. On another occasion all electrics began failing, no speedo, engine temp going up, gearbox won't change gear. End up sat on the side of the M5 stuck in 5th and can't start the engine. The alternator had failed; everything, including the throttle is electric, fan, Selespeed hydraulic pump, etc, so when the supply voltage drops things stop working. A recon alternator fixed that; once again not a Selespeed fault, could happen on any modern ECU controlled car.
So my personal experience of the Selespeed is good, but I would echo some others concerns around the availability of skilled or knowledgeable mechanics; the dealers are a waste of space and good indies who know the Selespeed are scarce. So its either learn it yourself, many owners have, or locate a good indie. If you are anywhere near Oxfordshire there is an ex forum member well known for his Selespeed knowledge (if not for his tact).
From what I can gather, early Selespeeds were trouble prone, later ones were better. I don't know when the change happened but I believe at least some of the problems can be put down to clueless dealers service departments.
Most of the issues I read about others having can probably be traced using a proper diagnostic approach and a good proportion seem to end up being a gearbox mechanical issue, maybe exacerbated by the Selespeed pushing gear selection hydraulically at around 50BAR.
Because of the rep Selespeeds seem cheaper; I think they're a good punt if you're prepared to DIY, otherwise expect to spend some time opening your wallet.
I wouldn't touch a JTS Selespeed, but that's because of the rep of the JTS, not the Selespeed.