As a long time jammer (this is my 16th jam), I've always done my games with GameMaker. But this time, since I was working with a friend who is new to game dev, we decided to go with Pico-8, and use only the cheatsheet to have a limited set of stuff to learn. This is our final game by the way:

It's a digging Tetris-like in which you have to remove layers of soil while not disconnecting the entire chunk, you can test it out here: SIRTET.

Anyways, this is an unsorted list of impressions about working with PICO-8 in a jam:

• Using only the cheatsheet as a reference is surprisingly easy. Everything except understanding the music creation interface was pretty much self-contained in a single-page A4 PDF.
• It's much less batteries-included, so if your game has heavy reliance on collision detection and the like you're gonna have much more boilerplate code than in GameMaker.
• Things being 128x128 and the minimal function set makes iterating pretty fast. It's like having a guardrail against yourself going too crazy with the scope of the game.
• The 16 color palette also works in the same way, making your games scope much more manageable.
• Having no shaders, adding juice to the game is a bit more hacky. In our case, we used a custom particle system and screen shaking.
• The SFX making interface is wonderful. It's a pity you can't use external audios so easily (I'd love to use JSFXR or modulime.
• The music making was the most complicated part for us. The music tracks are made of SFX tracks, and looping bars is a bit convoluted. We ended up making 4 tracks of drums, 4 of bass and 4 of lead sounds, and using memory operations (bitwise ops and poke/peek at memory addresses) to randomly change the track.
• Exporting was joyful. A single command and you get a web version that works out of the box in any device, with controller support, landscape mode controls...

This time we had a very clear idea of what we wanted to do since day 1, so the creative process was very straightforward. After 4 hours of development we were already focusing on the fun cycle. In general, I think that's what made this game so addictive vs previous ones.

Our final conclusion is that PICO-8 is really fun and smooth experience, but if you need anything more custom or advanced (physics, external graphic design, more complex palette, external music...) using another software is the way to go.