This topic has been covered by Scamp very good. I'd like to share my thoughts on it anyway since I'm in this situation right now.
It's tough. Choosing a new creative partner is difficult. You kind of know what you want, and what you don't. You meet a lot of diverse people, you do some trial working. Some stuff is good, some isn't. There are so many elements to it. First, it seems easy to find someone you get along with. It's much harder to find someone who you great work with. I think a good partner should be able to teach you a thing or two, and the other way round as well.
On one hand you want someone different than you, but then you need to be on the same level when it comes to liking certain ideas. Discussing and arguing is cool, when there is an agreement at some point. How different can you be? How similar should you be? It seems to be a rather difficult balance act. But to get that sweet spot right is crucial.
Having a partner who shares the same view on great advertising, is that good? Do you go with someone you do smart work with, or work that is more creative and 'different'. It all depends on where you want to end up agency wise.
Taking enough time for that process is important. Don't get me wrong, you shouldn't be sitting around for months and waiting for the perfect match to come along – always keep working. Work with as many people as possible, get the big picture. Be proactive.
Another point currently discussed with many of my friends, does the old formula or art director + copywriter still work? What about two art directors? Isn't that very helpful if you're going above the line where execution has to stand out and not much writing is necessary? Can art directors write headlines? Yes they can. Can they do it as good as a writer? It depends. I'm very curious about opinions on that.
Frankly, once you've been working with someone for a certain time and then you're on your own, it's kind of fun. But it gets boring after a while and you see the point in having someone on your creative side again.
In the end, it's all down to your gut feeling. A person who listens to you, and you like to listen to, is very valuable. Someone who is able to inspire you, is as well. It's and endless topic. Share your thoughts if you like.
Let's see where the creative partner search takes me in the end. I'm curious. It's exciting. Change is good.