OK, so... boring post ahoy...
The JTS came out in 2002, it was Alfa's (Fiat's) first direct injection petrol engine and one of the first anywhere. It made decent power for the time, 165bhp from a naturally aspirated 2.0 which is better per cc than the 220 from the V6 or the 145 I think from its predecessor, the 2.0 Twin Spark. It's also much lighter than the V6 lump which in a car like the GTV is important. If it had all worked out beautifully it might have been a better choice.
The direct injection bit is the problem. Normally, with fuel air mixture injected in the intake, the fuel washes away any carbon deposits but with direct injection this wash effect is lost. Carbon buildup is exacerbated by modern environmental measures like the exhaust gas recirculation system. This is a problem that applies to all DI engines. Much later engines introduced some old fashioned port injection as well to provide some washing.
The net effect over time is this, which are my mechanic's photos of my 156 JTS during a rebuild:
Over time this reduces power output but also blocks injectors and causes expensive problems,
and it's said that very few cars actually made 165bhp from the factory in the first place. I got mine remapped and dyno'd after the rebuild and it eventually made 170bhp, but it was expensive in itself. Even after all that it still drank lots of oil and I was contemplating getting more work done until someone stole it and wrote it off.
Additionally mine was from 2005 and in I think mid-2003 they introduced a modification to the cam timings to reduce some of the exhaust recirculation, so earlier engines are even worse.
The JTS was decent when it worked, and if you own one and have money there are good things to be done with it, but if you are making a fresh choice, go for the V6. It's probably more reliable and, more importantly...
...the V6 up until this generation (not later ones) is a Busso creation that sounds and looks brilliant, and it's a regret I haven't yet owned one.
One of our neighbours bought one of these when he retired for fun runs out. Marlin kit car, based on a Marina. 
But he loves it, it does have a Rover 3.5 V8 engine and Rover gearbox. It sounds awesome when it fires up. Think he paid about £5k for it. He spent last summer rebuilding the engine, new cams etc.
www.pistonheads.com
