I'm a no gi grappler, just tried gi rolling for the first time on Sunday. I felt like a complete beginner again. I got choked out with my own gi like 4 times. After that I protected my neck. But man, it was tiring!
2 things I really like the gi for:
-2 of my instructors and I ran drills for about an hour, and the technique drills, especially armbar from the guard, were WAY harder with a gi on. There was so much friction between the gis compared to no gi, it provided resistance for my armbars. As a result, I was getting sore and fatigued from armbars. I like this a lot; I feel like my armbars are going to be slightly improved.
-In the winter, my MMA gym is FREEZING (for training anyways). I open it up on Sundays, so I get there before anyone else at 10:30 a.m., and it's tough to get warmed up enough. The mats get really cold. But in my new Warrior One double-weave gi, I was breaking a sweat in no time. Gi grappling is great for the winter when it's really cold.
BUT...
-I've heard stories from either side of the fence as far as whether or not the gi helps you with no gi. The Gracies, and many Sherdoggers, say the gi will make you more technical, while a friend of mine on these forums who I will leave unnamed, and Eddie Bravo, believe that the gi is an anachronism and that it will only further complicate your game.
I have yet to see if gi training will make me more technical, but for the time being, it's going to be no more than a once-a-week thing to train with a gi. I have to focus on no gi grappling, takedowns, kickboxing, clinching, plus mixing that all together; gi training is going to be more supplemental.
The gi game is so different to me, because all my experience in grappling has come from wrestling and no gi. I'm very used to keeping things really tight, and using grips like overhooks, underhooks, arm drags, etc. The gi game is slower, you make more distance between yourselves, and you grip the gi a lot.