I agree with the general sentiment that jantar Mantar is a confusing nomination.

Foremost, it's Right in the middle of jaipur old city. So it's hardly justifiable to make it a whs on its own. This is a clear case of duplication, something I've found India is particularly adept at. 

Furthermore, the site is quite hard to interpret at the moment, with very simple and lackluster explanations. 

The there's the OUV. The instruments though certainly monumental, weren't that advanced for the time, and in fact their size is somewhat unjustified. The astronomers in charge of the project weren't adept at architecture and the soil has given up under the heavy weight of their sun dials making the instruments unprecise. all in all I would say jantar Mantar is more of a unique expression of the science of astronomy in India. In that sense it is truly remarkable and unique (though a few more jantar Mantar exist in other Indian cities, like Delhi) and I'm happy it's part of the list. 

The visit is short and pretty ordinary. Though the giant sun dial makes for great photos.