I recently evaluated the 3D engines available on iPhone. This is the list that I compiled, plus some commentary.

• iTorque (commercial) is the well-known Torque game engine available for iPhone. It comes in three flavours (2D, 3D, Studio) and professional and indie license, although indie is not yet available (scheduled for October 15th). Since Torque is quite popular, seems to be well documented, has a huge community and several products using it have shipped, this would be a good bet, but from my point of view, it’s overpriced.
• SIO2 (free) is a game engine with a bunch of cool features like physics and scripting, plus a sophisticated tool-chain around Blender. Here’s a YouTube video showing the engine in action. The only thing I don’t like about it is that so few people seem to be using it and that you need to display an 10 seconds video when using it – I’d prefer to add them to the credits.
• Oolong Engine (free) is an engine written by Wolfgang Engels, one of the computer graphics gurus around, if you ask me. It makes heavy use of PowerVR formats and brought Bullet physics to the iPhone. Unfortunately, besides some examples and a mailing list, there’s barely any documentation available.
• Unity3D (commercial) is a huge, fully-featured 3D game engine which was announced to be coming to the iPhone in March 2008. Unfortunately, there’s not a single sign of it yet.
• Shiva (commercial) doesn’t seem to be an engine but more an authoring tool, but I list it for sake of completeness.
• Living Arcana (free) is a project which features an iPhone port of an existing 3D engine which will be available under LGPL. Unfortunately I didn’t find this port.
• Rumor has it that Irrlicht comes to the iPhone but I cannot confirm that yet.

Needless to say, I’d go for Irrlicht once it’s available. Still, I’m interested in your experiences. Which engine do you use?