I have a 15" Bailey on a Next Generation frame. I love it. The speed is not an issue for me. That said, there are a couple of things that I would change. Also know that I have made at least a half dozen changes to my frame, too. So, it just may be me.
1. The handles are too high for me (I'm 67, 5'6" and prefer ergonomic things). DH is changing mine to a bicycle handlebar and wiring my switches in the end of the handlebar.
2. I would like a up/down switch. I have figured out how to do that w/just turning the speed dial all the way down. So that's not that big an issue.
3. I removed the cover from the front of the machine for more visibility.
Regarding the tracks on the frame. Change them to 1/4" round stainless steel rods ($80). They work so much better. My frame had plastic tracks, first set cracked in less than 6 months. Grace sent me new ones, they cracked in less than 6 months, then Grace wanted me to pay for new ones. I went w/the stainless steel rods. Best solution. Nice smooth motion.
If you get it, watch Jamie Wallen's YouTube video "Taking the stress out of longarm tension". Thanks to that video, I have very few tension problems. Before, I had lots of tension issues.
I started w/a Pfaff 9" GrandQuilter 1200. Would I like a bigger, more bells and whistles system? Sure. I make at least a couple of dozen items a year and I hate tying quilts or pushing them thru my domestic machine. So this works for me.
By for now, I'm going downstairs to quilt.