[Galaxy S I9000 Q&A, Help & Troubleshooting] Shanakin:
There are two major bugs for the international SGS line.

The first one, lag, can be fixed pretty easily, if you don't mind rooting your phone and generally doing things that void your warranty. The main problem is to do with the file system used on the phone. It may seem fine at first but it bogs down after a while and a week or two later can be pretty unusuable. Although this varies from person to person, and a lot of the newer firmwares are a lot better than the old ones. Still this is pretty easy to fix permanently if you have a problem, just flash a kernal supporting voodoo lagfix or universal lagfix and then convert the filesystem.

The second major problem is the GPS. User experience varies very greatly for this, some report their gps working perfectly all the time whist others claim it to be literally useless. The problem seems to be a combination of hardware and software and there are a lot of fixes which have widely varrying success rates and may work perfectly for one person but be useless for another. In my experience the gps is probably not reliable enough for navigation but will do alright. Sometimes it locks to an accuracy of 5m - other times 5km. If you consider GPS an important function, then I would probably have to recommend you look elsewhere.

Gingerbread, the latest version of android is available for our phones, but it has a few bugs in it. So some people prefer running the later version of froyo which are very reliable and there is lots of support and extras for. However as gingerbread is more commonly adopted it is starting to take over and get many of the extra features that are available for froyo.

Alternatively, the phone now has official cyanogenmod support after a long time of it being unofficial and too hard to make work as an official version. This will enable you to run the latest and greatest android versions for some time to come but at the moment it drains battery much faster than samsung based roms.

As for gorilla glass, it's certainly a great thing to have, and much better than normal glass, but by no means does it make your phone invincible to scratches and cracks. Some people may still prefer to use a screen protector, some people do not, it's really up to you to decide that. However I've seen far too many expect gorilla glass to mean invincible and they get a big scratch on their 3rd day or something and then complain too much. Having tried it with and without a screen cover, the plain glass does feel a little bit nicer, but if you get a good quality cover it's almost unnoticeable. Just take care of your phone either way.

Now as for sound, out of the box it is pretty average. However it has a high quality audio chip and its average sound output is due to rushed sound drivers. Supercurio has essentially reworked the sound drivers and has made a huge difference to the sound making it very high quality. There is a free version and a paid version. The Free version gets pretty much all the same stuff but you have to use a custom kernal with voodoo sound built into it. The paid version only requires your phone to be rooted and it can inject the voodoo sound drivers into any kernel. The best bit about this is you always get the latest version of voodoo sound without having to flash anything new. Even if the kernel you use has an old version of voodoo sound, using the paid up you will always get the newest. The paid version also gets the latest new features before they are brought to the free one.

EDIT: One last thing, try and have a look at the display model. The Galaxy S uses a pentile matrix sub-pixel layout which some people find looks funny and they cannot stand it. If the screen looks fine to you then don't worry about it. It looks fine to me.