The Mellotron did have a particular tonality, but was notoriously unreliable and made the player adopt a very specific playing style. I don't see the point of an accurate hardware emulation unless you absolutely have to have it to cover things like
Knights in White Satin or
Strawberry Fields perfectly.
The
GForce M-Tron is a great sample-based software emulation of the Mellotron. The
Tweakbench Tapeworm is a decent freeware emulation.