I have been using Maruti Zen for the last 3 years. I found it great on the following:
Powerful engine
Good fuel economy in its class
Balance
Pick up
Low on maintenance
Low cost of spares (comparison in its class)
Economic (in its class)
Light weight (.8 MT)
It has scope of improvement on the following:
Look (Although new Zen is really good looking, but still has long way to go)
Space : Less leg space, boot space and less room on back seat
Ground clearance : Ground clearance is not sufficient for Indian Road condition and steep bumps
Features : If you see Xing or other Santro car, you will find the same much feature rich
Light : Indica, Santro headlights are much powerful compared to a zen.
Height : You have to crawl in to the car, need to increase height.
Even in car there is less head clearance.
Scope to make it more sturdy.
Although with all the limitations, Zen is Zen. Its merits weigh much more then demerits.