I have a U-Trak trackball hooked up via a USB Opti-Pac, and my spinner is a mouse hack hooked up via the PS2 port. On the desktop, both 'mice' are working. The X axes will actually compete against each other; if you spin both X axes in opposite directions, the cursor will bobble about, not sure what to do, until one stops spinning (spinner always wins).
In MAME, they work the same way, so as long as you don't bump one while using the other, there's no conflict at all. The only issue I've noticed with this setup is in Typhoon 2001 (a Tempest 2000 clone). On startup, the game adopts the mouse settings for the first mouse used. I have the settings the way I want them with the spinner, but if I happen to move the trackball after starting up Typhoon and before I touch the spinner, then the control is absurdly fast. Restarting the game then avoiding the trackball until I touch the spinner clears this.
As far as the flight sticks go, I bought a couple of Saitek Cyborg sticks off of Ebay for like 8 bucks each. I stripped the base casings, which were mostly empty space, then mounted the guts and sticks in the panel. A few month later I discovered Randy's TRON and Satan's Hollow handles, and replaced the Cyborg grips. They look great.
Now, they are analog sticks, so they don't behave in certain games as you assume they should just by looking at them. For the games that use flight sticks, it's pretty easy to bump the sensitivities and get the feel you want (I've got a pretty low sensitivity set for using them in tandem for Assault, for example). The only thing I absolutely cannot get them to work for is the TRON light cycle levels. It's impossible to get an analog stick to behave like a perfect 4-way, but then I have a Pac-Man style 4-way right next to the flight stick, so for the light cycles I just bite the bullet and use that, with a panel button for speed control.
They also don't feel like crowbars that you could use to drag the cabinet around, but then I got two sticks for less than the cost of one Happ (and the Happ has no TRON or Satan's Hollow style).
There's a lot of pics in the thread linked in my sig.