I have not personally used any of the megachromes, but I have seen them in person. They tend to have a bit of a purple tint and are very crisp and sharp. I know that is not very helpful since words don't matter much here. My friend who uses them said that they output was what you might expect from another HQI bulb - more in line with Radiums over Hamiltons since the HQI Radium uses more wattage.
If fPhoenix 14k ever disappeared, Megachrome 17.5 would be on my list. They look different, but I like it the few times a year that I see them. I also have a ton of m80 ballasts.
The megachromes used to be HQI only and I have no idea if that changed, or not.
For those who have not used Halides in a while, the heat is not what it used to be. Radiums still get pretty hot. 14k Phoenix never really did. The new hamiltons, giesemann and RB bulbs have been made in the last half-dozen years to keep IR heat lower. Plus, nobody uses grow bulbs anymore, except for maybe Adam at BC, and most reef bulbs are not as hot as those were. I had a 250w HQI Phoenix 14k over a 60g cube in my office and the tank would not even heat up a degree with a simple fan in the summer months... and we do not even use the AC. There is a link to this tank in my signature.
This whole heat argument kinda reminds me of the people who think that carburetors are somehow just nasty and full of polution. I have a 1972 big block olds and I tuned a carburetor to have emissions simialr to a brand new Toyota Camry (and less than a new v8 SUV) even though I have no cat converter. Sure, it took work and would need new jets, rods and timing at different elevations, but here at 5200 feet, it gets 20+ MPG and is low emissions. Sometimes the old thoughts are just not right anymore, especially when you know what you are doing.
Edit: 15:00 here on a Wednesday. 7 MH on over my main display - 5x 250w HQI and 2x 150w HQI. 6x 300w Eheim heaters are on too... so I guess that my MH are not putting out enough heat.