In my career, I've been through the acquisition process many times, mostly on the "acquired" end, but occasionally, on the "acquiring" end.
To be honest, most of the time being acquired hasn't ended well. Think of it this way - you joined your original company for a variety of reasons. Perhaps it was a startup, and you liked the idea of being involved in the formative stages of a company. Perhaps you wanted to work in a small shop. Perhaps your expertise was highly valued to help "get things going". Perhaps you wanted to work for a particular manager. Now, a new company comes in and takes over. Chances are high that many of the things you previously valued will change. At least some of those changes will not be in your favor - larger company, more established, less chance to make an impact, working for a different manager.
So often, lots of people in the acquired company leave. Some leave immediately, others later. Some leave on their own, others are let go.
Still, an acquisition can provide lots of new opportunities. The new, bigger company may have more chances for advancement. Often there is a big transition effort in which new leaders emerge - perhaps you. Sometimes the post-acquisition environment has bigger market opportunities, more chance for your product to meet the needs of new people. Sometimes you can get exposed to newer technologies. Often, there is a cash infusion that makes far more rapid growth possible. Many times larger companies offer superior benefits. Sometimes, just having the new company name on your resume has value when you eventually move on.
Many times there is a financial incentive to stay around for a while. If you have stock options in the acquired company they will typically be either purchased outright or transitioned to stock or options in the acquiring company. Sometimes you are offered a retention bonus to agree to stick around through the transition.
The upside of sticking around is that you have access to all these possibilities in the new company without having to apply for a new job. It's a new chance that is just waiting there for you and your colleagues, you are all in it together, and the acquiring company usually wants to make it work and will try hard to help.
The downside is that you don't really know what is in store for you and it's not a path you have actively chosen. It's going to happen for good or bad.
I believe that if there are viable attractive opportunities available for you now, similar opportunities will be available for you later, after you have had a chance to see where the acquisition is going. I almost always advise that you stick it out, at least for a while, and see if it is something you really want or not.
That said, immediately looking elsewhere isn't considered bad form at all. I have had several candidates tell me in interviews that their small company was being acquired, that they didn't like where it was going, and that was their motivation for wanting to leave. I understand that.