Yeah I have heard a lot of mixed reviews about that.
By the numbers though, from what I've seen with PAR testing, a 3100K CMH outperforms a 400 W Hortilux HPS by a small margin. Those are the top of the line $70 HPS bulbs, but they didn't test any of the more economically priced ones. However, with the longevity considered, a HPS bulb has about a 10k hour lifespan, while the CMH bulbs still have 80% of their original output even at 24k hours. Meanwhile they're about the same price as that $70 HPS.
I'm not convinced they'd replace a 600 W hps but I don't have practical experience with either... But everyone I've heard that talks about them says that a 315 W CMH will yield the same as a 600 W HPS. I don't know if they've done side by side comparisons, but that's basically what convinced me to switch. That and the fact that CMH is supposed to run as hot as MH, and not as hot as HPS, which I kind of needed for environmental control.
Anyway, I haven't seen a huge difference in yield, and I use to run a 400 W HPS with just generic Apollo bulbs. I mean, most of my crops with the CMH have been smaller, but I've also been growing in less than optimal mediums, in small pots, etc. On the other hand, the bud I've grown with CMH has been noticeably better. It's just as dense, not fluffy or anything like that, but it is MUCH more resinous and sticky. Having talked to a lot of older growers, that seems to be down to the blue spectrum. Red spectrum has gotten a lot of hype as being the best to grow bud with, but that's mainly just because everyone got use to HPS delivering the most yield. I have actually had a lot of people asking me how I'm getting shit so sticky and saying they haven't seen stuff like that in 20 years etc. I think that has a lot to do with HPS having been the de facto "best" for about the last 20 years. My last cycle especially, I used the CMH along with some T5 so there was a lot of blue, and that shit was so sticky you could press it up against glass and it would just sit there.
So anyway I'm blabbing... OH yeah the weirdest shit with CMH is that plants don't seem to stretch under it. Like, say you took a MH or an HPS, and you stuck seedlings under it and you put that light as high as it could go in your tent. Your seedlings would stretch and fall over right? Not under CMH. They stay short and squat just as if they were growing out under the sun. It's bizarre, but I think it's just because if you look at the distribution of CMH's spectrum, it's a lot closer to what plants naturally expect from sunlight than HPS or MH, and perhaps a lot of LEDs.
I got a "conversion kit" type of setup where they give you a little adapter so you can plug the CMH lamp into an ordinary mogul socket, so that you can take your current MH/HPS setup and just pop the CMH right in place of the lamp and ballast that's already in place. Cost me $200 and I've already used it for 3 pretty good cycles.
