I think for most people BMI is a good indicator of whether or not they need to lose weight or can lose further weight. There are some extremes where it isn't reliable (i.e., people with a high body fat % and little muscle and vice versa), but I think those are rare.
Having said that, once you get into the mid to low end of your BMI range, I think your body fat % is a better indicator of how realistic your weight loss goals are.
Understanding Your Body Fat Percentage gives a nice summary of how to use this measurement. Here are a couple of web sites that calculate it based on your measurements and/or weight:
freedieting.com,
BBLex. They come pretty close to the body fat % my trainer calculates for me using the caliper method.
Also, I'm not a nutritionist, dietician, trainer, or any sort of expert at all, so take what I say for the amateur opinion that it is.