Definitely the best expansion for the Sims 2. And so much more than just businesses.
Now, I won't go in all the details, but one interesting thing is that the level of competence of a Sim with a certain perk shows. Bad cashiers just look terrorized when they have to serve customers.
Another interesting part is that you see your other Sims shopping in your business. This is a great enhancement to storylines in general. Okay, they'll end up with total random items in their inventories, but still, it's fun.
A salon is one of the easiest and funniest business. You can change Townie's looks and enjoy seeing them walking with it... One of my Sim drew whiskers on an angry customer. He walked around with it for two generations!
But the funniest part ever while owning businesses is to give to your employees... uniforms. The more random, the more fun! Monkeys, pirates, knights, you name it. However, I experienced a bug where, in community lot, Sims changed face at the same time than clothes, but you might not experience that problem.
Finally, the businesses can be used to other means than... just owning businesses. For example, you can make a church and have Sims flowing in only on Sundays... So, yes, Open for Business is a really fun expansion, and really easy to use! You just need a little patience and hop, you have a successful business!