Starting here, starting now There are a few ways to protect yourself and keep your career moving forward even if you are unhappy with your current job. Such talk is sure to eventually get back to the person and it will undermine your relationship with them and anyone else who hears such talk. If someone hears you talking about a user behind their back they might just begin to wonder what you say about them when they aren't around. Everyone has different skill levels and different levels of maturity, but they are still deserving of your respect on a basic human level. This is even more important to remember if you are dealing with outside clients.
Nothing can sour a business relationship more quickly than idle talk from the staff.
You don't want to responsible for a client taking their business elsewhere. If you don't get fired outright, the reduction in business could put you on the road to layoffs. It is always important to remember that, in most cases, you are merely the focal point for a user's frustration and feelings of inadequacy. It has very little to do with the quality of your work. If you are doing the best you can in your current situation then you must remember that fact.