IMO it looks great but there are draw backs.
It traps fish mulm like no tomorrow, it is very messy to clean and trim because it grabs onto the substrate and it wont come up without a huge mess.
If you want the look of a moss carpet the best way I have found to do it is to seed a bunch of stone, I used small pieces of lava rock, and just line them all up on the bottom of the tank.
The nice thing about the lava rock method is that you can remove all the stones from the tank, throw them in the bucket and give them a haircut "outside" of the tank. Before putting them back you just give the substrate a quick vaccum and lay it all back in for another month or two.
Also with stone you can get neat elevation changes by stone size.

roud:
Here is a photo of a 10 gallon that I started the moss bed in on lava rock as an example. The size of the stone you use will give any look you choose. You may regret putting directly to the substrate after some time.
Click on the photo for larger image
