Pros
They don't lay anyone off when the acquisition takes place, and will compensate for current benefits they don't offer with an equitable pay raise.
Cons
After about a year, which is just about enough time for them to completely destroy whatever culture your firm used to have, the layoffs start. After inflicting severe morale deflation to your office environment, the layoffs just intensify. In the case of my firm, Stantec turned a once thriving, professional family-type atmosphere where you mattered as a person to upper management into a cold, depressing, stressful, impersonal workplace where you're just an employee ID number generating profit for stockholders. Loyal long-time employees will find themselves suddenly subject to the whims of impersonal bean counters thousands of miles away, possibly in a foreign country.