Is a SanDisk 16gb ultra memory card good for a Nikon d3100?

Just got a Nikon d3100 and I want to know is a SanDisk 16gb ultra memory card is good for it?

Check your owners manual. SanDisk is generally a highly regarded brand. I would be surprised if it wasn't compatible. If the camera shoots video, get the fastest card you can--not sure where the "ultra" falls in that respect. Fast meaning read/write speed.

The brand is fine and the ultra will perform very well if you are shooting Full HD video, but the size is just too large.

Four 4 gb cards is a much better idea. Putting all your "eggs in one basket" is just asking for disaster at some point.

What it does is make you lazy. You will forget to copy all your image files to your computer at the end of each shooting day.

It is very important to do that and then when you know that all the images are safely on your computer, you need to format the card using the format feature on the camera.

Page 177 of your manual tells you what brands are approved by Nikon. If you don't have your manual, download one from Nikon.
http://www.geofflawrence.com/photography_tutorials.htm

Yes but it would be wiser to get 4 or 8GB cards and then buy them 4x or 2x, depending on which one you go with.

Here's an SD Buying guide - http://www.the-dslr-photographer.com/2010/02/buying-an-sdhc-memory-card/