Let me jump in here with my expereince in After sales service with Fiat and Maruti. I think I am qualified to talk about after sales service since I have experience with supposedly the best (Maruti) and the worst (Fiat and/or Tata-Fiat) companies.
Maruti
My problems with the SX4 are well known on this forum. Both Vitesse and S.K. have absolutely no idea what they are doing. Having done two services I can say that as long as the dealers have to do routine services, everything is okay but when it comes to diagnosing the problem, Maruti dealers are as good or as bad as any other. They all keep harping on "This is EPS and that is how it is designed" line.
The good thing about Maruti service is the look and feel of their service centers and the friendliness of the staff. The staff is willing to listen to you and if you make a noise, they take you VERY seriously. This effect, I believe, trickles down from the company.
Because Maruti takes customer's and customer complaints VERY seriously, the dealers do the same too. When I faxed a complaint to Maruti, they were extremely quick to respond to me. A service engineer was arranged to be sent to a dealership of my choice at my conviniant time. I was having problems with Vitesse and told Maruti that I don't want to take my car there anymore and the engineer should come to S.K. The day my car had to be checked at S.K. the service manager from Vitesse also drove down all the way from Worli to Vashi. The official line was that he took self initiative and came down to see if my car was attended to properly. Irrespective of whether it was his initiative or Maruti's arm-twisting, it was a very good gesture. Of course, nothing came out of the service engineer's visit as the car was found to be behaving normal. But, the service engineer, Vitesse service manager and S.K. Customer care manager were with me for almost 3 hours and offered a test drive on their demo SX4 and any other EPS equipped car if I wanted. To their advantage, even the demo SX4 behaved the same way as my car so their arguement that the car is normal had more weight

.
If I had a similar problem in my Palio would Fiat have taken the same pains? Seeing their track record,
I don't think so. Fiat Service.
I am among the few that found Fiat's service better then TATA-Fiat's but that could be because of lower volumes at Fiat exclusive dealerships. Anuroop was simply the best and I rate them higher then even Hyundai dealers. Of course, there was never any issue in my Palio so my only experience was doing routine servicing. The common thing between Maruti dealers and Fiat dealers was that the service advisors were with me when I went to pick up the car post service. Maruti service advisor at S.K. saw me off till I drove off in my car, Anuroop service advisor walked me to the car and as soon as I opened the door, he wished me bye.
Tata-Fiat Service.
Wasan and Fortune. Less said about Fortune the better. Wasan fairs marginally better. I got my Palio serviced on Friday. No complaints apart from routine service so can't say about their repair capablity. Big problem was in the attitude. I had a 9.40 appointment, service advisor attended to me at 10.30 am! Other advisors were sitting and chatting but refused to attend to me because "Vinod is your assigned service advisor, sir." If Vinod was busy with another customer, it was fine but at least he should come and tell me that he will be with me shortly. When I went to pick up the car, Vinod was nowhere to be seen. I had no idea where the car was , where to find the keys, what to do etc. When I put that down in the feedback form, a woman picked it up and Vinod appeared suddenly explaining the job done, fluids changed etc. After that he requested me to change the feedback form. I didn't. Why? Because, I believe these things should be done because thats how customer service should be. Not because a adverse customer remark will lower your CSI scores. To be fair, Maruti service advisors are the same.
Spares Avaliablity
My Palio hasn't needed too many spares. It hasnt had an accident so I never had to order non fast running spares. But, I have had problems sourcing the S10 specific muffler. Mind you, dealers have always had GTX mufflers in stock, it's the S10 mufflers that Fiat has forgotten. Twice fortune placed a order for the S10 muffler and both the times Fiat sent them the GTX mufflers even though their dealer had GTX mufflers in stock. Apart from this little blip, I have had no problems. But, it's irritating that Fiat has forgotten the S10. The car looks weird shod with the GTX muffler.