Did you try the Mellotronic M3000 app? It's been out for a while now and it is incredible. Spot on.
The korg people released their classic boxes as iPad apps and did an amazing job. I don't know how they did it but everything is reproduced on the screen.
I have the Korg Electribe and the ims20 and both are perfect. The MS-20 has the cabling and everything. Sounds great through real speakers (great time killer through the car speakers) or headphones (airplane flight time killer) and everything records and exports. I didn't get the vocal synth (model?) but it is a temptation. There must be a bunch more by now.
Kind of makes one want to pick up a physical box, but those are all hundreds of dollars. I really like playing with the Electribe.
Just took a look at that korg app after I read this. That thing looks bad ass, I like how it's patchable. I may have to grab it at some point.
I used to have an electribe but sold it. I miss the shit out of that thing, so fun to play with. I liked how you could feed audio through and chop it with the sequencer. With some tweaking you could really get that thing into out there zone.