T-pace is great, it improves your aerobic endurance. Some Mile Repeats at T? Hell yeah
I-pace is great, it makes you able to run hard while out of breath. Some 1000s at I? Hell yeah
F-pace is great, it improves running economy and top speed. Some 200s at F? Hell yeah
R-pace sucks it just gets you dead legged and tired why in the world do people do quarters and 600s at R pace. I mean 300s sure but at that point why not just do F pace