Let me start off by saying that we're naturally resistant to change. Change is scary, and most of the time we assume the worst. But I'm just not able to get my head around any good redeeming qualities of Allies in the new edition of 40k.
I think it's great if you and your friends want to do a fun game with some themed allies, but I can't think of anything but broke. As far as I know, the codex were written in isolation of each other. Never did they think about how an allied retinue would help. In my mind, outside of Grey Knights each codex has its benefits and drawbacks. Allies basically trump that.
Visually it bothers me as well. Very rarely will we see themed up allies that match the parent list, it's just going to be borrowing from each others armies to get new combos. It's a sad way to get cross army sales from GW.
I really hope I'm wrong in my extreme distaste of allies and it will only be used as "with opponent's permission" or something to that degree. Otherwise if it gets too over the top, I'm sure most tournaments will end up having no-allies only tournaments.
We'll see one way or another in less then a week. Until then, I need to get started painting on my beastmen!