InSSIDer is still a very good, 'quick check' type of tool. Since it's free and uses the laptop's built-in NIC, it's always available. However, using the laptop's built-in WiFi NIC means scanning times are slow (~4 seconds) and readings depend on the quality of the NIC, the OS drivers and the laptop antenna - lots of variables.
I always carry my Ekahau (Proxim) NIC around with me and fire up Ekahau even when I need to do a quick RSSI measurement. The ability to select which channels to scan and the universality of the NIC in the industry means that the results are consistent and repeatable. With recent ZD OS and RuckOS versions, Ekahau even shows the number of clients associated to the BSSID and the utilization of the channel.
watch this space for Ekahau as v8.5 brings spectrum analysis and then soon after that there is something new coming to the fold, which will make you rub your hands in excitment - cant tell you as I am bound to secrecy.
In terms of clients and utilisation , they will be offering a cool way to graph clients and utilisation against all AP's captured during a live survey, so you can revisit and look at the values obtained from the IE's.