We will be in Vienna the week before Christmas and I was considering purchasing the Vienna 72 Hour Card. Expedia have it for USD18.17. I gather it provides umlimited travel in Vienna and entry to selected museums and some restaurant discounts. Any comments appreciated as to whether it is worthwhile?

No need to purchase through Expedia. You can purchase it at any U-bahn or train station from very convenient (and in english) machines. I would personally not buy the Vienna card but I always get the pass for all the public transit. If you are staying a week you might want to consider buying the 8 day pass. Vienna transport is the best. most efficient, and safest I have ever experienced. The discounts offered by the Vienna card are minimal and I am not sure about free admission to the major museums.

We bought one each on our last trip and I figured that the pass paid for itself rather quickly. We rode from our hotel to Schönbrunn Palace, back to the hotel, from the hotel to Die Volksoper. then the next day up the Prater and then to the Belvedere. The convenience of getting discounts and free rides without having to fuss with tickets was very good.

We ended up riding the street cars just to see where they went. Probably we would not have explored so much had we had to buy separate tickets. But once the fee is paid and you can ride all day every day, the temptation to wander around on wheels gets to be very tempting.

In a phrase: I thought the pass was worth the cost.

True, but just by buying the transit pass on its own gives you the same benefits.

A regular 72 hour card costs 12 euro( about 14 dollars at todays exchange rate) Operaman is correct to buy it in Vienna not by Expedia.
You valididate the card on your first ride on any bus, streetcar or U-bahn.
The ticket is then good for exactly 72 hours.

The Vienna Card is good for the same 72 hours - cost 16.90 euros( 19.75 US Dollars.)
I would look at the discount brochure before you buy it to be certain you may visit those places that offer the discounts. Not all museums do.

The 8 day card costs 24 euros and is valid for more than one person. You just validate card each day for the number of people you want to ride.
This can be your best buy if you will use 8 days worth of ridership.

Public transport will take you everywhere in Vienna. It is clean and safe with frequent service on most lines.
When you buy your ticket at a ticket booth ( at most major stations- example- Stephensplatz, Westbahnhof, Scottentor) get a map of the system. It can be a big help.
Be certain to take a ride on streetcar #1 or 2 when you arrive. It is a good way to be acquainted with the city and many major tourist sights.

Thanks to you all for the feedback. Looks like the card is the thing to buy since public transport in Vienna is good and buy it when we get there.

The Vienna card seemed to me to offer 10% discounts at restaurants that weren't meritorious, or free desserts of someone else's choosing. Thus why pay more than for a 3 day transit pass?