GameMaker 8.1 is definitely better than 8.0 - not by much, but it is. The reasons not to upgrade seem somewhat trivial and more like a desperate attempt to find something wrong for the sake of it.
Nope. Decreased stability and even font kerning are actually important issues. If I would use GM8.1 from the start, Cinders would be a flop, with around 30% of sales having to be refunded. I wouldn't call that trivial. GM8.0 is tried and works on almost everything. For a commercial (or just popular) game that's a huge thing. And you can always port to GM8.1 later should it prove to be as stable as 8.0 in the long run.
I have used GM 8.1 for a long time with several projects, including a very large one, and have experienced absolutely no instability. I'm not certain about your issues with font kerning, but in my experience GM 8.1 actually improved my use of fonts because a bug is fixed where font characters are cut off by a few pixels(a GM issue I have despised for years).
I was very hesitant to move my major project of 2+ years to GM 8.1 and paid close attention during the transition, but found not a single problem. Granted, I'm sure our games are set up very differently. But for me, all I saw was improvements, and my user base did not have a problem with the transition.
There may not be a demo for GMS yet, but it is worth the money.
That's pretty subjective. If you're just making freeware hobby games then it's not worth the upgrade at all since Studio is a buggy mess right now.
Studio is getting regularly updated but right now the only reason to upgrade is if you're making HTML5 and mobile games.
I'm talking from a commercial perspective. GMS is a product made for developers that want to make money with their GameMaker games, hence the much higher price tag. It is not for hobbyist developers.
GameMaker Studio is for exactly that: "making HTML5 and mobile games".
Aside from performance boosts and more stable cross-platform support there's no real reason to upgrade from GM 8 to GMS if you're only a hobbyist developer. I would not have bought GMS and all the modules if I didn't plan to make my money back ten-fold (and so far I have

)
GameMaker 8.1 is definitely better than 8.0 - not by much, but it is. The reasons not to upgrade seem somewhat trivial and more like a desperate attempt to find something wrong for the sake of it.
as teegee said, *not working on 30% of computers* is not trivial
I've been following Cinders, and we can all tell it's not your "normal" GM project - it's massive, and includes large art assets. Now my major project has been in development for a long time and is quite sizable, certainly larger than 99% of GM projects, and I experienced not a single issue with my transition to GM 8.1.
I do know what problem teegee is talking about though, I experienced it myself when I used huge background texture in one of my games. It ran fine for me, but I received reports of it not loading for a few others (this '30%', perhaps?). I was notified that it is because GM 8.1 does not support such large images, and I'm sure there are reasons for that change (ironically, probably to improve stability).
Was that a big deal for me? Not really. I altered my game to use a smaller texture and everything was fine. For Cinders, teegee probably has the option of separating his high res asset into smaller sprites. Is it easier to stick with GM 8.0 though, instead of doing that? Yes, most certainly.
So holding onto this "your game now won't work on 30% of computers" is not representative of everyone's experiences, and is something that developers CAN address if they experience problems (correct me if I'm wrong, teegee). In his case, sure, I can see that as a reason to stay with GM 8.0.
besides that, that thread i linked to pointed out several major disadvantages of 8.1 vs 8.0, such as compile times being twice as slow, empty game .exe files being 2mb larger, and so on. i don't know in what world you'd have to live in where those can be considered trivial
This response in that same thread counteracts many of the points raised:
http://gmc.yoyogames.com/index.php?showtopic=505774&view=findpost&p=3736908Compile times twice as slow? Not for me. Mine where much faster, and my loading times improved many times over.
Extra 2mb on empty files? Apparently not according to the above linked response. I didn't bother trying this myself, because when your project is already 50Mb in size an extra 2Mb is not going to kill anyone and yes, I consider that absolutely trivial in a world where patches alone are frequently 400Mb+.
let's just consider the 2mb larger file size. let's say you are a commercial game developer, and have to pay bandwidth costs for people downloading your game's demo. if you have 100,000 demo downloads, a "small" 2mb increase in file size for the demo means 200 extra gb of bandwidth that you need to pay for, which can cost you several hundreds of dollars. and that's being somewhat conservative, some games have millions of downloads for the demo version
I am a commercial game developer, and I have unlimited bandwidth on my web host and excessing bandwidth on my VPS. By the time a commercial game is so popular that its demo is receiving 100,000 downloads, I'm quite certain you'd be able to afford a little extra bandwidth.
Now I want to finish by saying that again, this is all just in my experience. My transition from 8.0 to 8.1 was excellent; much smoother than I anticipated. Will my transition from GM 8.1 to GMS or GM 9.0 be so smooth? Absolutely not, it's going to be incredibly painful.