I have most of the synth apps for iPad and iPhone/iPod Touch. This is one of a handful that I actually use.
Ive had Sunrizer (originally Horizon) for the iPad since it came out, and bought the XS version as soon as it became available. More screen space _is_ better, especially for playing the keyboard, but the iPad doesnt fit in my coat pocket. It was a real pleasure, when work was over yesterday, to pull out my iPod and savor the rich sounds of this virtual analogue synthesizer. I asked the developer what XS stands for and he replied extra small, like with t-shirts. So far as I can tell, however, the only thing thats been compressed here is the interface, not the sound. For the iPad version, the developer worked hard to fit almost everything onto one screen. With XS, the various functions are -- of necessity -- spread over several screens, but theyre all here, including the arpeggiator. (The arpeggiator on this synth is very powerful; for tips on using it, see the four-part tutorial at http://iosmusicandyou.com.)
A couple of minor bugs, to do with loading and saving custom banks, have been fixed in version 1.1, and some new features have been added: most notably, virtual Core MIDI channels (so that you can play the synth with another controller app on the same device, such as SoundPrism Pro), FM synthesis, and phase modulation synthesis. Its also easier to download new soundbanks; try "New Territory" for examples of how to use the latest versions FM synthesis feature. (The reviewer who complained about the FM "waveform" doesnt understand what hes looking at. His mistake is understandable, but its a mistake.)
There is one thing that newcomers should be aware of, but its a limitation of the hardware not the software. That is: the iPod screen is too small for playing a piano-style keyboard comfortably. This is especially true for chords, where adult fingers crowd the surface. Fortunately, Sunrizer has a way to program chords so that you can play them with one finger. It is worth reading about this in the manual to learn how this works. The doublestacked keyboard that another reviewer mentioned would also be welcome, but the wider screen on the iPod and iPhone 5 does help, and the latest version takes advantage of it.
There are already a lot of enthusiastic reviews for this app, but let me mention some features that seem to be overlooked.
1. Were used to the modulation wheel controlling one or two parameters at a time (usually a filter). With Sunrizer, the modulation wheel can move smoothly between a whole group of parameters (called "morph groups"): not just filter frequencies, but LFO depth and frequency, oscillator detuning -- just about anything thats a knob. For example, lets say you want to make the end of a phrase more emphatic. With the morph groups feature, you can increase the feature resonance AND widen the pad AND slow down the vibrato AND make the vibrato more intense, all with one motion. Its very easy to set up and, in performance, opens up a highway for expressiveness.
2. As of this writing (March 2013) there are now several excellent synthesizer apps for iPhone and iPad. But this one, Animoog, and Magellan have the best presets.
4. For Vangelis-style patches, be sure to download the web bank titled "CFA Vintage Electronica Virtual Analogue" by Constantin Aliferis. Sunrizer got its start emulating Roland synths, but can also do a convincing CS-80. The default "Blade Runner" patch is one of the best Ive heard on any platform.