I had the commonly reported poor WiFi signal and zero GPS reception on my iPhone 3GS . The GPS and WiFi had previously worked faultlessly, but then suddenly stopped. Initially I blamed it on software, but later realised that it was probably because I had dropped my phone at around the same time as the fault developed. So, I had to replace the WiFi/GPS antenna (a common cause of WiFi/GPS problems on the iPhone)..
I suppose the repair went OK - hey, everything now works!!!!!.... but it was a NIGHTMARE to do. The components in the iPhone are absolutely tiny - especially those 1.3mm screws - many of the components are fiddly to position, and then you have to hold them in place while trying to correctly position a tiny fixing screw. Younger folk would probably find it not too difficult - especially if the guide is followed carefully AND all the notes contributed by those who have followed it are read and understood BEFORE starting the job. But unfortunately, my 65 year-old eyes are really no longer capable of this sort of fine-detail work - but I did manage it eventually and I now have a fully-working iPhone with WiFi and GPS. I've no idea how long it took me in total - probably about five hours, much of which was spent developing techniques to make re-assembly easier. I found that the best time to tackle it was first thing in the morning, so I took it very slowly and on some days only managed to get one or two screws back in place.
Don't even start if you have problems seeing and handling tiny, tiny screws. Read the guide AND all the notes several times before you start. Be prepared to be beaten by this task because it isn't at all easy. Have a nice big magnet to hand because you will almost certainly drop some of the tiny screws, and I found that the only way to find them on the floor was by "sweeping" with a magnet from an old loudspeaker. Finally, have some Blu-Tak to hand. I found Blu-Tak was essential for temporarily "gluing" components out of the way. In particular I found it useful when holding connectors or ribbon cables out of the way. Take care, of course, that you avoid contaminating the phone components with Blu-Tak, and only ever use it on the "non connector" side of the connector on a ribbon cable.