Pros
The Research Assistant (RA) position is one of two methods for engineers to receive a Master of Science Degree for free. The other is a Graduate Teaching Fellowship (GTF) which I am unable to comment on. The RA works 20 hours per week on research while taking two classes each semester. You have a large amount of control over the way you go about your research although your advisor will step in and micromanage if you are not making progress. The work / life balance definitely leans more towards "life" than "work". If you take summer courses you can finish in 1.5 years. Overall the RA is a great opportunity to get a MS in engineering relatively quickly without accumulating large amounts of debt.
Cons
As an RA you are at the mercy of your advisor. Most RA's work more than 20 hours per week although I was lucky enough to keep my hours down. Upon entering a research program many MS students hope to deliver some sort of a completed project, whether it be a piece of software or a hardware component. The reality is that professors want students who are interested in learning for the sake of learning. If you are motivated by delivering a completed project this is not the job for you. Unless you plan on pursuing a career in the area of your research, it does not add much strength to your resume. If you plan on working in industry after you graduate, a GTF position will give you a more marketable skillset.