I've maintained GIV, IVSP, 450, GV, 550 all were very reliable.
The GV was sold after 10 years/ 7500 hrs, 3500 landings and was AOG once in that time.
Gulfstream spares and back up is first rate, I could always get a spare part delivered in Europe in 24 hours.
Gulfstreams are not cheap to purchase or operate, if you are worried about $$$'s then its probably not for you.
Having said that, if you are looking for a reliable, versatile business aircraft, then Gulfstreams are the cream of the crop.
Tony