Although it promised to be exciting, it ended being boring/dull/useless.
Any idiom for the above phrase?
It ended not with a bang but a whimper. (a slight adaptation of T S Eliot's famous stanza).
There are probably a slew of idioms for this. The first few I thought of were these:
It fizzled out.
It petered out.
It ended with a fizzle. [Not sure that this is really an idiom, but it's close.]
It was a dud.
It bombed. (def 2)
A single word suffices.
is a quasi-non-standard neologism since the 1970's. It was originally created as a snarky backformation for the other dierection from 'overwhelming', but it has since become accepted. For example,
It was a wasted evening at the symphony. The premier of the wunderkind's latest piano concerto was underwhelming.
If you're referring to something not living up to the hype, you could say:
All Sizzle, No Steak.
or
It failed to deliver.