I've used intra-workout products for years. I've used intrabolic, intra-aid, xtend, bulk bcaas, purple wraath, intrain, sizeon maxp and pre-contest. Currently, I'm using 2scoops of M.A.P. with some Green Mag in there as well.
IMO, it helps a lot with stamina, especially if you're doing fasted training and so didn't eat before lifting.
However, most of these products are drastically underdosed and overpriced. Again, that's just my opinion. For instance, I was using 4-5 scoops of intrabolic to get a benefit.
So I had to find a cheaper alternative. Now I pretty much just mix bulks so I get exactly what I want.
If you want to go for a premade product, though,
Sizeon and Intra-Aid were the best I've used.
Purple Wraath and
Xtend also give you a decent value for your money.