I enjoyed the Sumida River Fireworks Festival from the nearby Asakusa View Hotel. The only problem with the event was that I wasn't clear on the starting times, and I believe that there was some slight delay.
The event is a little spoiled too by the ten-fifteen helicopters that zoom around (presumably they are filming for television programmes). It makes it hard to get a decent photograph of the event, and I think over half of my photographs had a helicopter in them.
Still, the event is nice from a viewpoint, but perhaps quite busy as you get closer to the river. Also, the trains home are far too busy, that it might be recommended to take the short walk to another station (i.e. Ueno), if you are heading back across Tokyo after the event.
As for the fireworks, they were amazing, and some of the most spectacular I have ever seen. Definitely worth it, despite my complaints.