I've used nearly every version of GameMaker, since the beginning in 1999. Back then I was barely involved in the online community, but I was quite active with GameMaker itself.
There have been a few things I liked that were lost from version to version, but overall the gains have been far, far greater, so it's definitely been worth moving on to the latest GMS2 for me. These days, I can't even stand working in GMS1 or GM8 - I have tried, just to see if things were as good back then as I remembered, and it's been shocking how much has improved with later versions, and I didn't realize until I tried going back.
Even though the gains are far greater than the losses/downsides, I must be honest and say some things do make me pretty sad about GMS2 (though very few). One example of something I really don't like with GMS2 is the fact that resource names, functions, strings (RIP story-based games), shader script code, etc., are all easily visible in "data.win". It feels a bit unprofessional to me to have that data laid so bare, being easily viewable by even a layman who only knows how to open it with something like notepad. Granted, with some of those things, you can find workarounds to obfuscate, but not all (at least, not to my knowledge). IIRC, Studio 1 also had this issue, but it would export data.win in fewer circumstances, I think? It's been a long time since I exported in Studio 1, so I could very well be wrong on that. In any case, it's something I see as a big downside with GMS2, but it still pales in comparison to all the awesome things GMS2 allows me to do over GM8 and prior versions, or even Studio 1.
Of course, I didn't always move on to the next version right away. In fact, I was VERY stubborn with moving from GM8.1 to Studio. Eventually though, I warmed up to Studio after using it enough to get comfortable with it. Then, when GMS2 came out, I was once again thinking, "oh great, I have to relearn a lot", and I was ready to be stubborn all over again, but at the same time, I could see that it'd be worth it, so I forced myself to move on as quick as I could. I'm glad I did and I don't regret it at all.
So the only reason I would ever use GM8 again is if I was someone who was stuck with an older computer that couldn't run Studio 1 or 2, or if I had no money to buy GMS2 desktop, but already had the full version of a previous GameMaker. Even then though, I own the modules for mobile export in GMS1, but not in GMS2, and I honestly want to do some mobile development, but I'd still rather wait until I can purchase the exports for GMS2 instead, so even in this case, I guess I still wouldn't go back.
-
One last thing to note, thinking about the GMS2 workspace UI, which might dissuade some people from moving to GMS2 from previous versions, since that's one of the major differences:
I personally stay out of the workspace as much as possible because I have trouble keeping a good workflow when using it - it feels cluttered and hard to focus as I need to. But the good part is that you don't have to work in it much; there are other options. For e.g., I just right-click on objects and use "Open All Event Scripts" so I can have my pure and cozy GML fill the screen, which lets me follow the flow of code and focus so, so much better. That's one of the good things about GMS2 - you have a lot of options to work how you want. I think some people shoot it down without exploring their options.
For anyone still stuck in older versions, I encourage you to give GMS2 a real shot. Even now, GMS2 is more than worth upgrading to, and GMS2 is only getting better. I mean, look at the new updates in 2.3. So many of those things are great improvements (e.g. asset browser customization, in-line functions, structs [those "lightweight objects" you may have heard promised a while back], etc.). Full disclosure: I haven't tried the beta yet, I've only read about the features. But I'm planning to try it out very soon, and I'm very excited.