Are City Passes worth it?

I have purchased and used city passes from the following cities' tourism centers: Dublin, London and Rome.  However I decided they were not worth it in Barcelona, New York, Boston and New Orleans.
Why some cities and not others?
It depends on how long I will be staying, what sites are included and overall cost.  Look for it to include bus and subway fares included or heavily discounted.  Look for front of the line entry to the popular sites - so you don't have to wait in line.
Dublin: I was there for 7 days and paid approximately 60 Euros on sale - note watch for discounts on these passes. Almost all cities offer discounts if you sign up early for their newsletters.  I find just before one of their long-weekends due to national holidays they discount them.  In Dublin I pretty much did everything included in the pass so it was worth it and it included transportation. 
London: I was there for 5-days and did the three-day pass for about 99 pounds - again I got about 300 pounds of value as I wanted the more expensive sites - Windsor castle and it included travel to and from London's Paddington station, Westminster cathedral, St Paul's cathedral, tower of London, boat tour to Grenich etc.. What was great was that I planned on doing 3 sites in a day and usually did 4 or 5 as I planned by location.  Some places I would not have visited without the pass including the hop on hop off and royal art gallery and stables. 
 Roma Pass- 3 days including transportation was great value for the Coliseum front of the line, museums etc. I highly recommend the Villa Borghese gallery but you must book in advance for a specific entry time.  Again I did some things I wouldn't have but the entrance was free (i.e. Ostia Antica). Note this one doesn't include the Vatican or sites in Vatican city.  I recommend for St. Peters you do the Capella - its an elevator to the top of the cathedral and then the views of all of Rome are the best.
Why not in Barcelona? - I really wanted the art galleries including Picasso, Miro, the national gallery so I bought an Art Pass for 26 Euros which included front of the line - an absolute necessity for the Picasso museum.
Why not the US cities? Mostly they don't include things I wanted to see in the short period of time they offered and the value to price didn't seem good.  It was cheaper to buy online discounted priced advance tickets for each of the museums.

The best tips I can give you is to plan ahead, sign up for the city pass newsletters and wait for sale/discounts and then plan your route in each city to visit sites close together each day to get maximum value.
Hope this helps.