Recently updated to Universal app status, SampleTank for iOS is IK Multimedia's take on the 'pocket DAW' concept.
It's entirely sample-based, with no synthesis functionality onboard beyond a simple resonant filter. It also lacks sequencing, so it's not quite in the same arena as NanoStudio, BeatMaker 2 and GarageBand.
Four instruments can be loaded at a time from SampleTank's library, which covers the gamut of acoustic, electric and electronic flavours. 136 sounds are included with the app itself (four of which are unlocked by registering), although another 400-odd are accessible in packs via in-app purchasing. These packs cost between £1.99 and £2.99 each, or £27.99 for the whole lot.
That makes it over 40 quid for the full SampleTank experience, which certainly puts it up there with the App Store's more premium offerings. The sounds range from impressive to passable - more than good enough for 'sketching' purposes.
Triggering sounds is done manually via the on-screen keyboard (which can be restricted to specific scales) and drum pads. Over 1000 well-performed preset patterns are included, serving as a useful grab-bag of starting points.
Basic recording functionality is built in, including variable loop length, overdubbing and quantise, but there's no sequencer, so you're not going to be producing full tracks with SampleTank. There are also no effects, so the only sound-shaping on offer is the aforementioned filter.
SampleTank's strength lies in its diverse sounds and patterns - it is undeniably comprehensive in terms of instrumental and stylistic coverage. However, beyond that the app is all very 'light' and preset-driven, enabling no real control over composition or sound design, so we have to question the high price.