The app is very simple to use and comes with eight sounds. Three are ready to go and five are available via in-app purchase at $0.99 each. The included sounds -- Roof Garden and Waterscape -- feature rain and thunder. Waterscape adds running water of some sort, likely a stream, while Roof Garden features overhead protection of sorts. The rainwater sounds very realistic in both recordings.
My favorite sound is Desert, available via in-app purchase (note that you can preview sounds you haven't bought). It features a wind of varying intensity that really does sound like it's moving past / behind you. The thunder sounds are also quite effective.
Navigating Thunderspace is very easy. The main screen features a play / pause button and a lightning icon (more on that later). To browse the list of available sounds, swipe to the right (tip: swipe to the right again for a tip on using the app).
From the play screen, swipe down to reveal six buttons: Restore (for restoring purchases), Support, Share (let your friends know via social media), Rate, Play intro (a nice introduction) and Gift. Here's another tip: pull down again for a weather-related factoid, like, "The average speed of a raindrop is 17kph."
Now back to that icon. What's a thunderstorm without a light display? Tap the icon to engage lightning mode. Every now and then, as your virtual storm rages, your iPhone's flash will briefly fire, simulating a bolt. It's cute and kind of fun.
Thunderspace is similar to Naturespace, which also provides 3D audio to the iPhone, but Naturespace offers many other types of audio, not just thunderstorms. I like the tight focus in Thunderspace, however, and they're doing a good job. Grab a copy, put on some headphones and reeeeeelax.