Pros
SEIU does great work and I've very proud of the work I did while working with them. They are at the forefront of the labor movement, and they are leading the way to raise the minimum wage. I had the opportunity to work independently in the field most days, while feeling supported by my supervisors and coworkers. It can be a pretty relaxed work environment.
Cons
Ask a lot of questions before you take a job with SEIU. Every local differs a lot. Some of the higher ups at the International are evil. They see organizers as disposable, and they are union-busting their own staff union. There can be a lot of turnover and great people get let go. There can also be an unprofessional work culture (inappropriate office romances, people talking about drugs and possibly doing drugs while on the clock), but again this depends on the local.