CSM Ally?

How about?

It's hard to say if it's a good idea when they're about to get a new codex. Whilst it's theoretically a great idea to ally an army that works very well in sixth (DE) with the first army with a codex specifically written for sixth, they might not mesh that well. I'd wait and see what the new codex brings. Their stuff won't be scoring for you, so you'd really be just bringing them for their firepower or raw killing ability. Obliterators will probably be as safe a bet as ever though!

I tried using them.

1 DP w/ wings, MoN, and the doombolt.
2 5 Noise Marines w/ Blastmaster in a Rhino
1 3 Obliterators

As long as you make sure everything is greater than 6in from each other you don't have to test. I was to fond of the Noisemarines they didnt do much. THe Obliterators deep striked in and took out some things, only issue with obliterators is they don't get overwatch because of S&P. I played the DP as a FMC instead of a JMC, it was pretty good, especially using the primaris power from pyro. The DP took out an entire squad of pathfinders in cover. I do hope they FAQ the DP or in the new codex the DP is a FMC with wings instead of a JMC.

I think the rumour is the CSM Daemon Prince will become a flying monstrous creature, like the daemons one, but that CSM will be losing the Lash of Submission (thank goodness). Presumably this is the reason they're currently still jump instead of flying, as a 48" lash would break the game!

lol unlike a flying daemon w/ pavane??? they dont even test for that one

CSM offer nothing that DE dont already have. DE shoot better, assault better, and have better vehicles. The only reason to get them is for some saves on your units, but desperate cant even score for each other, so I dont see the reason why you would ever team up with CSM. atleast Daemons offer amazing templates and some flying MC goodness (oh... and those amazing AP screamers now )