1. It's fine but it's not cheap. I think of it as the lazy person's option. It is significantly cheaper to do some research, plan an itinerary and use taxis and the metro. $60 per person per day buys a lot of taxi time.

There are options for a 48hr ticket, or a combination ticket with Abu Dhabi. http://eng.bigbustours.com/dubai/home.html

2. It depends on your tastes. Are you attracted to 'new' Dubai with glitz, glam, malls and towers, or something a little more historic and 'authentic' ? Do one each day and decide for yourself.

Whether you find it too hot for walking depends very much on where you're coming from. March will be high 20s - 30 C. But with taxis so numerous and cheap, no one walks far here.

The UAE is perfectly safe for everyone at any time of the day or night.