Avantages
The best reasons to work for H&R Block would be for the experience and knowledge you gain from working there. Client service, interviewing techniques, tax, accounting, and legal knowledge are hard to come by through education alone. You need experience to get your foot in the door at most firms, but can't get it at many places. This is a Fortune 500 company that will hire you if pass the basic building blocks course. From there, with a few years of experience and some certification (Enrolled Agent, CPA, JD, etc.) you are likely to land a job with a private firm or one of the Big 4.
Inconvénients
The major downsides to working at H&R Block are the lack of benefits, the ever-changing "schedule" you are to work, and the relatively low wages when compared to the rest of the tax and financial planning industry as a whole. You also have to deal with a wide spectrum of clients, some of whom are very polite and nice, while some are very arrogant, angry, and/or downright hostile. While this is a good training environment, it's not for everyone, and there are always people who start work knowing the theory but can't handle the pressures of the clients being "in your face" so to speak. There are also quotas for sales targets, new clients, and so on that must be met or the management will begin to ask you why you aren't producing.