For those who lean towards the competitive side, Allies offer many ways of making very powerful combinations (though not as many as the blogosphere would have you believe; lots of people don't know the rules for armies they don't play :D). Those players can min/max to their hearts content. For those that love painting and modelling Allies allow many unique and interesting conversions, combined force paint-schemes, crazy counts-as, etc. Even those that favor the story more than anything now have tools to recreate their favorite historical alliances. Allies offer something to everyone.
Personally, I'll be waiting to see what national and local tournaments decide to do with Allies. If they are in, then my dirty Necrons will definitely be getting some rusty Grey Knight buddies (modeled off of FW MKII armour types). As with all the other changes, I think everyone needs to calm down and wait to have the new rulebook in their hands before making any sweeping generalizations or declarations (note that I realize I just did this, haha).
I personally guarantee Allies will be at least this fun.