ZX Nostalgia was pulled from the App Store a while ago. I read some conflicting reports regarding why it was pulled, but a lot of non-conflicting reports about how awkward the controls were.
The best Spectrum emulator for iOS is iXpectrum, free from Cydia for jailbroken devices. It's nice on the iPhone but amazing on the iPad with the larger screen.
Landscape mode allows for touch controls in the border where fingers won't get in the way of the on-screen action and, for those games requiring more than half a dozen keys, portrait mode shows the screen at the top and a full rendering of the Spectrum keyboard below. Great for text adventures, or possibly even programming in ZX Basic if you're a masochist.
A built-in browser downloads freely distributable games straight from the World Of Spectrum archive, so you don't have to faff around transferring files from a host computer using SSL or a file manager.
I can't imagine any other emulator, paid or free, will come close to matching it. Unless you're religiously anti-jailbreak, or have a version of iOS that can't be jailbroken yet, it's the one to go for.