For many years, those who made the long journey to Lumbini found that there was not much there! The Maya Devi Temple, the Ashokan Pillar—that was about it. Things have changed since.
Plans were drawn up in the late 1970's by a Japanese architect to make Lumbini a place of both pilgrimage and tourist attraction. So now there's a Japanese stupa, a Burmese style pagoda, a Chinese style temple, some fairly formal gardens and a museum. The exact spot where Buddha is said to have been born hasn't been moved to make way for the newer developments.
But is Lumbini worth the effort of a visit?
The main reason people come to Lumbini (and in their thousands) is pilgrimage. Buddhists from around the world come here.
So, if you're coming here with faith and spirituality in mind, then the peace, tranquilly, and significance of Lumbini are obvious.
But, take a look at the map below; Lumbini isn't exactly close enough to other places generally visited in Nepal. It's on the Terai too, so it's a long way from the Himalayas.
However, it is possible to fly from Kathmandu to an airport about 15 miles from Lumbini.
For the more general "just curious" tourist, there's some nice parkland, monasteries, and the main sights of the Ashokan Pillar and the Maya Devi Temple. But if Buddhism isn't your thing, then we'd find it very hard to justify coming all the way to Lumbini.
The air pollution, due to heavy industry in the area, particularly from India, is particularly bad.