I've heard they are emblematic of the wasteful public expenditure in Japan.

They are very good and add interest to the streets.

Mildly interesting for kids but when you look at the bigger scale they hardly compensate for the architectural brutalisation all around.

Like Wallace I think they are interesting and I like seeing the designs I haven’t seen before. I have no idea about the cost/benefit ratio of them. For those interested and that have the time, you can pick up trading cards of a lot of the designs from places around Japan.

https://www.gk-p.jp/mhcard/

Pretty? Yes. Waste of taxpayers' money? Also yes.

They're very nice in themselves.

Do they cost much more than non-decorative ones and is there any particular reason for it if they do? They are made of the same material. It can't be that expensive to make a casting mold.

Obviously it would be nice if the same care and attention were paid to other parts of the urban landscape.

kohakuebisu

Do they cost much more than non-decorative ones and is there any particular reason for it if they do?

Whatever they cost is fiscally wasteful, if the existing manhole covers they replace are functionally in no need of replacement.

I have a good collection of photos of them from all over the country, so I guess I like them.

They are great.

Much better than falling down a hole.

I've never thought of them, which is somewhat shame for me as a Japanese citizen. Manholes are naturally blend in our everyday life, but I need to rethink of it's importance.

Do they cost much more than non-decorative ones and is there any particular reason for it if they do? 

Yes. And the reason is because of political favors to whoever gets the contract.

The manhole covers show local features, which is fascinating. I love them, myself.