HSS is High Speed Sync. Each manufacturer has their own term for it. But, basically it allows you to use shutter speeds faster than the camera's x-sync speed.
With normal flash operation, your 5D is limited to a maxiumum shutter speed of 1/125 second with Anti-shake on, or 1/180 second with Anti-shake off.
If you try to go any faster, the flash is not synchronized with the shutter (so, you end up with the flash firing after the shutter is partially closed, ruining the images).
HSS pulses the light so that it has a better chance of proper illumnating your subject, versus trying to time it so that the flash fires when the shutter is fully open and before it closes). The downside is that this pulsing greatly reduces your flash range.
A good example of when you may want it, is for fill flash when taking portraits in brighter light outdoors, using larger apertures (smaller f/stop numbers) to help your subject stand out from distracting backgounds, where shutter speeds would be too fast to sync an ordinary flash.
Do you really miss focus assist lamp in your typical shooting situations?
Nope, I never liked 'em. They give away your intent and make it harder to get candid photos if your subject sees an AF assist lamp.
Are there conditions when I could really use one? Sure. But, I'd prefer it if the AF sensors worked even better so that I never did need one. ;-)
Also, unless light is very low, the AF sensors work pretty goood on the 5D with a bright lens. I've managed to get AF lock in conditions with light so low I needed to shoot at 1/5 second, ISO 3200, and f/2.5 with my 28mm f/2
Of course, I'll admit that I had some trouble getting lock with lighting that low (and that's proibably an understatement). ;-)
But, a bright lens can go a long ways (since it helps a camera to see better for Autofocus purposes).