My rating above is not so much for the mosque, but for the river cruise, which is highly recommended if you don't have much time to spend in Kuala Terengganu.
Board the cruise at the Taman Tamadun Islam (Islamic Civilisation Park) jetty.
You can see the Crystal Mosque, the various islands on the river, the esplanade on the mainland, while getting a quick history of the area from the river cruise guide. It's also a great way to spend a hot afteroon -- enjoy a nice river breeze!
Tickets are RM15, I think and the cruise lasts for just under an hour. After the cruise, have some snacks at the riverside cafe at the Taman Tamadun Islam -- surprisingly, the keropok lekor, deep-fried squid, etc was delicious!