The common rim that FC owners are excited about is the 17x8" rims found on the GT models. The benefits are that these rims can be found used quiet cheaply, look better than the stock FC rims, are substantially lighter than stock TII rims and they are 17" tall (which seems to be the primary concern of many). Their weight is actually surprisingly close to many expensive aftermarket rims that cost a premium for their touted weight savings. I have the 16x7.5" rims pictured above. Beyond the practical differences, I think they look better than the 17" rims.
I weight a rim with a worn out 225/45/16 Yokohama ES100s mounted on it. I found that it weighed 34lbs. The Tire Rack states that new ES100s weigh 22lbs. They also state that new tread depth is 10/32". To err on the side of caution I'll guess that the lost tread weighs approximately 20% of the new tire weight, or about 4lbs. I believe this 4lbs number to be high so I'm also using it to aid in countering any potential uncertainties of my measuring equipment. Thus the weight of the Mustang rims would be 34lbs rim & tire - (22lbs new tire - 4lbs tread) = ~16lbs! This is a far cry from from the stock S4 TII rims which are 24lbs or the S5 TII rims which are 18lbs. In other words 24lbs of unsprung weight can be saved over the stock rims!
Oddly few FC owners actually seem interested in the fact that the 17" Mustang rims are 8" wide which will improve performance when using wider tires and allow the use of tires wider than on the stock 16x7" TII rims permit.
Personally I am not a "rim person". I think far too many people place too much importance on rims, especially considering that most FCs are not in their original condition. Also, the 'visual appeal' of 17" rims over 16" rims does not excite me. The 2000 and similar years of Ford Mustang GT and Mustangs with "Sport" package also came with 16x7.5" rims. All of these rims were essentially the same and have a have a removable center section that covers the lug nuts. Some of these center caps have the classic Mustang logo. The far less popular option was to have five holes that make the lug nuts visible. With this style of rim there are two different finishes. I am not sure how to qualify the difference but I will say that the finish I think looks best is typically on the slightly older models (eg. 2000) and those are the ones I have. I happen to think that these rims are the best looking Mustang rims available. The 16x7.5" rims can be found for a lower price than the 17x8" rims. While I do not have data to back up my assertion, I also believe that it that the 16x7.5" weigh less than the 17x8" rims (this should stand to reason) and still retain some performance advantage over the stock rims TII rims with their increased width.
An important note with Mustang rims is that their offset is lower (32mm) than the stock FC rims (40mm). This offset means that the rims will sit further out in the wheel well, giving the wheels a meaty look. The disadvantage is that it may be possible for the fender to hit the tire when full suspension travel occurs. I had this happen to me when running a larger than stock diameter tire (determined by the width and aspect ratio which relates the height of the sidewall to the width of the tire) and I was on a lapping day when I went onto the grass at 70mph. Those who have been to a roadcourse can probably testify that the grassed areas are not smooth. Such conditions are probably unlikely to occur on the street. I have not had any problems with my 225/45/16 ES100s even during off course track escursions.