Does your area freeze in winter and how old is the home? Yes it is a good product.

you don't need to spend that much money.

any membrane is better than no membrane.

level-ness is not the subject here; your mentioning it means to me that you have a long learning curve still to go. If you have no time, spend your money and hope your contractor is competent and capable of making the right decisions for you.

The house and floor are from 1980. We usually get a winter low in the 20s.

I understood that Ditra is intended to address flex and shifting in the surface underneath the tile. Do cement basement floors flex much? What other issues would the product address / should I be looking at?

You probably do not need it. If it is a basement and there is no movement in the cement, you are more than likely good to go without. open basements, etc that get freezing that crawls under neath the cement causing some minor lifting or flexing and the ditra can do wonders to prevent tile issues.

for membranes, see the following companies' web sites.
Noble
Custom Building Products

Post again after reading up.

If the concrete is cracked now, it may still be moving. and no anti-fracture membrane is going to stop the tile from cracking.

Over concrete, Schluter Ditra is really overkill.

There are other products that will work just as well and not as costly.

You can use Schluter Kerdi, over concrete

NAC has some stuff too...
http://www.nacproducts.com/ECB.htm

Here is a good link for you.
http://www.tileusa.com/antifracture_mfg.htm

Ditra is available for ~$1.50/sf from Home Depot; your contractor should be able to get a better rate on that. It should not require much more labor than installing hardibacker on a job that small. I would say your contractor is fluffing their bid.

Ditra may have cost more FOUR years ago..........

In my opinion, If the cracks are less than 1/8" and no evidence of vertical movement, I would specify a crack isolation mortar (various manufacturers make it). This would not require extra labor to install and no extra time, other than the usual curing times. Instead of spending $125 on mortar, you would spend $250 - $400. Ditra is worth the money if you needed crack isolation, waterproofing, and to meet a certain height requirement to transition other flooring materials. Your contractor is right to recommend something for crack isolation on concrete and there are many good options available and some have been mentioned all ready by others.

What do you think, is Ditra worth the money?

It's a good product but there is much better preforming options with less thin-set restrictions. Why not prepare the floors with Ardex 8+9? The cost is lower and Ardex 8+9 is also rated as a crack isolation membrane where Ditra is not. Ardex 8+9 is waterproof as well without all the banding work Ditra requires.

ARDEX 8+9: Concrete sub floor

This is the first coat of Ardex 8+9 over a concrete sunroom floor here in Vancouver. Crack isolation membranes should measure three times the length of the tile. Most crack isolation membranes are between 3'-6'. Since this room will get a 3' wide tile I needed one huge sheet. This is where the cementious memebrane from Ardex excels.

I also like that there is no voids under the membrane for little silver fish bugs to crawl around in and make homes. I find silver fish a problem in many Vancouver high rise condo's and try and stop them from making new homes on my projects.

Does your answer change if we install slate rather than porcelain?

All the more reason to use Ardex or NobleSeal CIS. Both those tile choices will want to see a modified thin-set used.

Nice work John,

How did you prep that slab prior to the 8+9...?

Before any membrane goes over a slab , in addition to cracks the installer should also be looking for moisture content in the slab and understand if it has a vapor barrier below .
(in addition to and Unlike Johns Above grade example )

We do use ditra on occasion but not for any serious crack isolation and never slab below or slab on grade. For cured slabs 30-40 days old min. We prefer true crack isolation membranes and the best thinsets for these applications like a submersible flexible highly modified thinset.