I don't like them as much as lychee. They have a bit of a nuttier flavor I think.
Depend on the variety, most variety are much tastier than lychee fresh off the tree, however they ferment/rotten quickly when ripe due to the sugar content.
You will never find the best variety that are almost seedless, because they are treated like treasure and usually consume by the locals (this variety normally fetch 3-4X the price of the varieties that we find here in North America).
There is also a practically seedless rambutan variety that can be found only in Thailand and it is absolutely the best rambutan ever (more fragrant and sweeter than the regular rambutan, and also fetch 3X price premium over that of regular variety).
<---- Grand parents had almost 4000 hectares of fruit orchards, and rice field (unfortunately the communist took it from us in 1975).
[edit] I mean hectares instead of acres.