Slow cookers are great, but they are only as good as the recipes and techniques you use. They are considered a "convenience" and as such many recipes try to be too convenient and end up sacrificing flavor. Many slow cooker recipes skip browning the meat and just have you throw everything into it all together. Sure, you end up with a an okay product but not a great one. Take the time to sear your meat, sauté or caramelize your vegetables, etc. just as you would if making a braised dish the old-fashioned way. Secondly, I think slow cooker recipes often overstate the time the food need to cook. They do this as slow cookers are touted as a way to cook while you are at work. In my experience recipes that call for 6-8 hours of cooking, tend to overcook all but the toughest cuts of meats. Most times 4-6 hours is plenty long enough unless you want your meat to completely fall apart.
Overcome these problems and it is a great tool. Sure I prefer to do my braises, stews, etc. in the oven or on the stove top, but as both me and my wife work, this isn't always practical, nor do I feel overly safe with our oven running for hours on end while we are not home. We use our slow cooker regularly and we love it, but we understand its limitations and make smart choices about what to cook and how to cook it.