That's a bit of an open ended question. Which tiptronic gearbox? Technically, Tiptronic as a brand name has only been used by Porsche and Audi. Those boxes, ageing now, are fine as long as you look after them and keep the oil regularly changed. 

 

If you mean more modern automatic and DSG-style dual-clutch gearboxes, the answer is two-fold. Generally speaking, the autos are hugely reliable (especially the ubiquitous ZF-made six-and-eight speed gearboxes) but with DSGs, the jury is still slightly out. We've heard no major horror stories but they are much more complex than a conventional manual or automatic and need to be cared for properly.