It totally depends on the city in question. For Glasgow and Liverpool, the HOHO's work very well (I was very pleasantly surprised). For most other cities, they don't.
I'm fond of the "tourist trains" that they have in many smaller cities, which are not HOHO, but make one loop around town, using a vehicle with narrower cars than a regular bus, so they can go into the old sections. The one in Toledo, Spain, for instance, not only looped through town, but took me to a vantage point just outside for great photos; it would have been impossible to get to that spot without a car. These are also cheaper than a HOHO, so I haven't lost much if I don't like them.
I agree that for Leipzig, you can walk the central section from the train station, with no need (on a day trip) to take either local trams or a HOHO. For the Hague, the attractions are separated; Rick's book will have directions on getting to them from the various arrival points (there are two main train stations, as well as a tram from Delft).
Also remember that on a day trip, a taxi ride or two can be very worthwhile, precisely because it means you don't have to learn the local transit, and because you get door-to-door transit, which saves time. I often take a taxi from the station to my first sight; in Ravenna, Italy, this not only saved hassle and walking, but got me to the mosaics ahead of the tour groups.
I also agree that for a day trip, a "city card" is rarely worthwhile. Again, though, it depends on the particulars of what's covered on the card and how you intend to spend your day. Some are almost impossible to make pay off; others can be a good deal if you're getting an early start and doing a lot (for instance, a walking tour, a boat tour, and two museums). City cards can be a better deal for more days, since the cost per day is lower, but even then, you have to do the math carefully, and remember that there are only 24 hours in a day and you can only do things when they're open, so you may not do as much as you think you will when you buy the card.