Zero Hour is a fantastic addition to an already great strategy title. The additional units open up more strategic avenues during play, the generals challenge is an interesting and welcome addition to the single player and the unique generals make multiplayer games very interesting indeed.
Online play is fast paced and largely centres around economic harassment with plenty of skirmishing over supplies and tech buildings to be had. Unfortunately quite a few of the maps are poorly designed for agressive play and allow "bunker noobs" to engage in turret spam fests and what not but a good player would deal with this situation quite easily so it's no real cause for concern.
There are a few balance issues to be ironed out, like S&D rockvees, gamma buses and what not, but on the whole there is plenty of fun to be had in both single and multiplayer. The bad news is that EA have officially declared that they are no longer supporting Zero Hour so the balance issues that exist at the moment are unlikely to be ironed out but don't let that put you off, it's a fantastic game and the online play is enormous fun.
All in all I would say it's a more than worthy addition to any strategy fans collection and if you already own C&C Generals and don't yet have Zero Hour then it's a must buy! If you own neither game you can buy the deluxe version which contains C&C Generals and Zero Hour for about £20 now, which is a steal to be honest.
What are you waiting for? Go on! Buy it! :D