Gone are the days of expecting someone to stay at the same company their entire career
Average is 5 - 7 employers over the course of a career
No longer the stigma attached to being laid off since many were impacted by corporate downsizing or restructuring, the dot.com bomb, etc.
The red flag to employers is numerous quick changes, appears to be lack of focus, lack of commitment, etc.
Everyone can make a mistake - better to admit it and move on, have story to tell about it and what you learned how you do things differently now to ensure that you make the right choices, I only stayed a short time because..., should definitely not have more than one of these situations on a resume or it brings your decision making skills into question.
Must have a story to tell about the changes - should never bash previous company or bosses - the world is too small, it can come back to bite you, also be careful not to explain moves in terms of problems or changes at the company since employers value employees who can solve problems and adapt to change and possibly lead change, they will shy away from potential employees who run from change or challenges.
Don't fudge dates or falsify your resume in other ways trying to cover up a short stint, most employers will do employment and education verification and you could lose an opportunity by not being honest.
The advantages are quicker career progression in some cases and often increased pay.
The disadvantages are many - gets more difficult to sell yourself, those looking at your resume will question your loyalty and long-term commitment, sometimes the short term gain is offset quickly by lack of stability, if you don't stick around long enough to experience the true business cycle in an environment you are also short changing your learning and experience, you may miss the very skill you need to develop for the future, also lose the opportunity to be developed for future growth opportunities within the organization since most organizations prefer to promote from within.
While most employers we work with know that most employees don't stay for life, they value longer assignments since that gives them better return on their training investment, they benefit from people who develop a broader knowledge of the business and they can develop talent from within.