I travelled on this on Tuesday, and was surprised at the number that were there at the same time.
I crossed from North (Royal Victoria Dock) to South (Greenwich Peninsula) at around 11.00am, and there must have been around thirty to forty people in the queue. On arrival at the other side, there was probably a similar number waiting to cross from the South, although how many of them were on a one way trip (like me), I couldn't say.
Here's the rub, though .... I'd say that 100% of the people I saw at both ends were tourists. Now this is fine if it can be sustained, but can it? From a very brief look around both the north and south areas, there doesn't seem to be too many large businesses immediately near to either terminal, although there are quite a few a short distance away (i.e. Canary Wharf etc.).
I decided to make the crossing before I saw this thread because I like travelling on this form of transport. However, having done it, I'm not planning on doing it again - unless I go from South to North en route to somewhere else, that is. If you like this sort of thing, I'd recommend it!