Engine and transmission parts are standard GM LSx parts (regardless of LS1 or LS2). The engine has been out since 1997 in Corvette's and 1998 in F-Body's and used in several vehicle lines, as well as cast iron blocked versions are the engines in all the trucks. So yea, it's reliable. Each engine (LS1 and LS2) has it's own specific set of quirks though.
LS1's like to leak oil out the main seals, and a porosity issue with the front and rear engine covers, whereas LS2's like to burn oil. Early LS2's (and this was in all vehicles it was put in, not GTO specific), had some oil pump failures. LS1 is a cable driven throttle, LS2's are ETC. This led to issues with the pedal assembly causing the vehicle to go into "safe mode". Most of those should have been fixed under warranty by now. LS1's have the dash cluster "freak out", notably on cold mornings, where the gauges just flip out and go crazy. This was due to a short in the cluster and then a short in the harness that fed it as it went through the firewall. All have lackluster HVAC systems (a common complaint is the defog on rainy days), slow power seat mechanisms, and many have suspension issues from the get go (How they were strapped down on the boat to be shipped here), and in general, most find the cars' suspension to "soft" for a car of such capability and spend time upgrading brakes (regardless of year), and suspension. This also applies to the G8 too. LS2 GTO's got better brakes from the factory, a slightly stronger 2-piece driveshaft, the cosmetic changes and a few color changes. All in all though, it is the same car underneath. So don't get hung up on OMG, it's only an LS1...
Where mechanical parts get expensive is after the transmission and then those suspension parts. Why? No other vehicle in the North American market shares them. The driveshaft is a 2-piece trunnion supported setup with a rubber "Companion flange" that it bolts to on the pinion of the diff. My STS is similar, but not the same. The differential is completely different than anything in the U.S. and it's part of a whole rear IRS setup. Upgrading diffs, to say a KAAZ unit, can be quite costly. Then start adding in stuff like cryo treats stubs and beefed up halfshafts, and parts get expensive quickly for either car. We've already touched on the suspension breifly, but again, there are only two real main supporters of aftermarket suspension parts for these cars. Peddars and at one time King. Another issue, and one I've been having for a couple of years now, now LS2 guys are starting to feel it, and probably in a couple years, the G8 guys will, is parts availability for regular, routine maintenance items. In many cases, the parts aren't even made anymore... As such, you're left to your own devices. I faced this with cooling and heating system plumbing last year, and ended up creating my own hoses/junctions. Why? They parts are no longer made and supplied down under, much less availabe to be shipped here...
Body parts are where it can REALLY start to get expensive. Why? Some 30k GTO's were produced and shipped here over three model years (Actually GTO's are more rare than G8's). As such, there are many parts available in general, much less available in GM SPO's North American network, which then means it has to come from Australia. Shipping big body parts across the Pacific ain't cheap... So what we've seen, is a rash of GTO's with what many would consider "minor" damage getting totaled, because the cost to get parts is astronomical. Hence my analogy of scratching it totals the car.
Aside from all that, the car is a very stout car in terms of solidity and assembly. Things are actually SCREWED together and torqued down. Not stupid Lee Press-fit tabs and plastic christmas tree B.S. like other GM vehicles. It's a nice feeling, but a PITA when modding because *shoot* is hard to get unassembled sometimes because it was put together so well at the factory.
There is a ton of info for reading over at LS1GTO.com (or LS2GTO.com... both will take you to the same site). It is the original site, the most tech heavy site, and has the largest database of info on the car. All other sites that sprung up are former members of LS1GTO who go mad and picked up their ball and left to create their own site in an attempt to outdo the original.
I'm coming up on 10 years of ownership with mine. Mine is one of the first batch of 465 to be built and shipped to the U.S. (Sep 03 build), and while I get frustrated at times with it, when I do drive it (once in a blue moon), it never ceases to put a big sh*t eatin grin on my face.
