So, I use Linux, and I\'ve been trying to find the time to get into game programming. I started out with Panda3d and had some pretty decent results and got a feel for many of th
Ogre3D
http://www.ogre3d.org/
Is typically named together with crystalspace and irrlicht.
Ogre and Irrlicht both are said to have a cleaner design than crystalspace so I wouldn't worry to much about problems with the latter.
I've spent some time working in the game industry. I'm also a Linux guy. I used Irrlicht to make a couple games, and used those as part of my resume, which helped get me get a job as a game programmer.
Irrlicht has a cleaner API, lower system requirements and works better across platforms than Ogre, in my opinion. I've had a blast making games with Irrlicht. It's also fairly lightweight (much lighter than Ogre), with a very open license for any use, commercial or otherwise.
Working with that engine did a lot to prepare me for working within the commercial game industry.
If you ask me, Irrlich is the best open source engine - it has clear architecture, good performance and requires lesser code to be written by programmer. I can not honestly compare Irrlicht with Crystal Space or Orge. I consider CS as mess of code, written by many different programmers in different manners - I am hard to imagine how to use it too (due to absolute lack of documentation). As for Ogre, it is not easier than D3D. I've examined different samples and fount plenty multyline fragments of code that are done with a single line in D3D. So, I simply can not see a reason, why to spent months learning OGRE's terrible API - if one has free time, I would advice to learn D3D itself. I can say even more - Irrlicht is better, than many commercial engines, for example - Torque (absolute lack of documentation, forces to start project over existing one etc.), Truevision etc. Of course, Irrlicht lacks some great features, AAA grade engines must have, but it is quite sufficient for smaller projects. If you have not BIG money to acquire Gamebryo and similar grades engine, I would advice to stick with Irrlicht - for first several projects at least.
If you're looking for productive game development, then my best bet would be to use Unity 3D. I started off by using Irrlicht but quickly backed out because of non intuitive tools and a lot of stress on programming. Ogre seemed to be even more complex.
Unity on the other hand is rapidly gaining grounds with each release. The recent Unity 4 is packed with a ton of features.
With very little game dev knowledge, I've managed to write my own flight sim engine in Unity Android. Even learned to write shaders easily.
Though advanced Unity licenses are paid, but they are well worth it. But you can always use Unity free edition to make commercial PC games. In all, game development is all about getting the hang of a game engine. Master any, and you will rule.