I've driven several different QTronic examples on test drives and I agree with Muz. The transmission seems a good match for the 2.4's characteristics, with a useful spread and appropriate gaps between ratios, and the shifting was super-smooth. If you're buying a diesel with auto trans, you're not to expect neck-snapping trackday-style shifts.
you're buying a 159 for its solidity and refinement - if you want quirky thrills, then you're better off with a 916 GTV or Spider like mine.
On a 3.2V6 with the QTronic, there was a low 'whorling' sound when moving away, reminiscent of a mega-mileage BMW (ZF box) which made me a little nervous despite the car's low mileage. Four other 159s (three 2.4s and one other V6) did not exhibit this sound. I also didn't think the transmission was a great match for the V6 - the wider rev range of the V6 meant that some of the gearchanges - particularly a double-ratio kickdown - seemed unnecessary.
The QTronic transmission appears to be bought basically 'off the shelf' from Aisin-Warner. It's made in Japan (probably with American input) and is used in a wide range of vehicles. I couldn't find any bad reports. As a six-ratio automatic with light weight (by conventional auto standards) and the euro-style lockup converter, it is designed to maximise economy. Specs show a reasonably small penalty of around 1L/100km compared to the manual (though, that is over 10%) and you'd expect most of the difference to be around town.
I expect the reliability would be at least as good as any other auto, and probably has an order of magnitude fewer warranty claims than Selespeed cars would have. I am a fan of Selespeeds (I work on them a lot, too).
The two manual 159s I've driven both exhibited a heavy and sticky gearchange that was not very pleasant - and there is the worry of DMF and clutch expense - enough to make me want the auto, particularly as my other half can't drive a manual. It's nice to be driven around occasionally