I haven't gone inside the mosque(not sure if it allowed) but whenever I am in town,I pass by for the mabuyus and mandazi(sweet snacks). You will always find women selling them on the street in the late evening. They are cheap and definitely worth a try. It is also a lively street because The Nation building and tribeca club are...
We have visited there with my girl friend. Because the place is very close to our Hotel. Firstly the responsible person hesitate to get us inside but after let us to look inside. It is a common mosgue and not special. If you visit there you may spend 2-3 hours not so more.
If you want to get some clothes... here is a nice place for quick easy bargain shopping. Make sure you negotiate the price at least down to a little less than half. Especially if you are a foreigner... they do take you for a ride... but you won't regret the shopping... especially if you like bargaining... :-)
I visited the mosque some 50 years ago as a child with my parent and visited it again on my recent visit. Inside it is still as I remembered it however outside the building there are many more shops and build up which has affected the appearance of this nice piece of architecture.