I'm going to use the Radeon HD 7970/R9 280X as my primary example here, as I have access to those GPUs and they actually work so far on all of the N-Factors...provided you have the right settings. Let's go over those here, just for fun.
Approximate Settings for Scrypt-Jane Mining
|N-Factor||Settings||R9 280X||R9 280|
|4||-I 18 -thread-concurrency 20480 -g 1||6.1MH||4.8MH|
|5||-I 18 -thread-concurrency 20480 -g 1||4.5MH||3.7MH|
|6||-I 18 -thread-concurrency 16384 -g 1||3.3MH||2.5MH|
|7||-I 18 -thread-concurrency 16384 -g 1||1.8MH||1.5MH|
|8||-I 18 -thread-concurrency 8192 -g 1||1.0MH||850KH|
|9||-I 18 -thread-concurrency 8192 -g 1||550KH||470KH|
|10||-I 18 -thread-concurrency 8192 -g 1||330KH||270KH|
|11||-I 12 -thread-concurrency 8192 -g 2||133KH||100KH|
|12||? (I missed this stage of YAC/YBC)||~70KH||55KH|
Starting at the initial N-Factor of 4, you can expect pretty amazing hash rates -- about 6.1MHash/s from the 280X, with TC at 8192 and intensity 18-20. Six days later you switch to NF 5 and your hash rate will drop to around 4.1MH, still with more or less the same settings. At NF 6 you'll drop to maybe 3.3MH, then NF 7 is going to be around 1.8MH, NF 8 drops yet again to 1.0MH, and so on. Along the way, however, you'll start to find that the settings that worked well for lower N-Factors no longer run without hardware errors at higher N-Factors -- and never mind what happens after about NF 12, where hash rates seem to drop far more than the 30-50% of earlier changes.
You'll need to tune the above settings a bit in most cases to get accepted shares and no HW errors -- in particular, I had a lot of problems not getting HW errors on the R9 290X at lower N-Factors. I also know that HD 5800 and HD 6900 hardware basically suck at scypt-jane (scrypt-chacha) for reasons I don't quite understand. Probably it's just the OpenCL code isn't really tuned for those older cards, or maybe the older hardware just lacks certain features, and that's fine. Still, it's interesting to look at how performance at the different N-Factors compares. As far as I know, scrypt-jane with NF-9 is basically regular scrypt, but hash rates are lower by around 20% compared to vanilla scrypt. At NF-10, again hash rates are lower than Scrypt-N by 15% or so.
One of the things I wanted to mention in all of this is that the initial stages of scrypt-jane are basically a headache for larger mining operations. Depending on how similar your hardware is, you could be looking at a few hours to perhaps a day or two of tuning and tweaking settings to get things running "properly" at each N-Factor adjustment. And if you're not paying close attention to all your rigs, you might come back from a weekend to find that all your PCs basically accomplished nothing after the latest N-Factor adjustment -- been there, done that, and didn't particularly care for the hassle, thanks! At least the rate of NF change slows down a lot at NF-14, but then at that point you're looking at rather low hash rates and there may be better coins to mine.
It's not too surprising then that most new "clone" coins that are moving away from vanilla scrypt are going with Scrypt-N (Adaptive-N-Factor Scrypt, as defined initially by Vertcoin). Sadly, VTC doesn't seem to be getting as much credit as it deserves, at least in terms of trade value, but then part of that might be due to the somewhat lackluster name and logo. There are at present at least seven cryptocurrencies I can name off the top of my head that use Scrypt-N, compared to six scrypt-jane options (though I'm probably missing some from both sides). The first scrypt-jane coins are already at NF-14, and frankly they've struggled since NF-13 with GPU mining; in a few months we'll see NF-15 and then three months later we'll hit NF-16. They might become predominantly CPU-mined coins at that point, which may or may not be a bad thing.
In short, Scrypt-N makes more sense to me from many perspectives. It skips the early (and somewhat chaotic) low N-Factor stages, it defines a more systematic progression from one N-Factor to the next, and in my experience at least it runs better on several generations of hardware. That doesn't mean it will necessarily succeed (see Betamax vs. VHS), but as more and more scrypt ASICs arrive and GPU miners look for greener pastures, I suspect the majority will end up with Scrypt-N (or something new). What will that be? I'm not quite sure, but as always I'm keeping an eye on things. If you'd like to subscribe to my email newsletter for more information on what I'm mining, you can find the instructions on this page.