So question is this..

I'm keen to get a flash so I start playing with flash photography and wondering if I should get a second hand or new 430ex II or get like a sigma flash? Do I lose anything other than TTL?

Depends on what you're talking about mate. If you want to start getting into OFF-CAMERA FLASH then you'll want a manually adjustable flash and not something that only does TTL (Like lower class sigma's, sunpacks, nissins etc..) that way you can dial in the power you want and adjust. You can always get a fully TTL unit (Like a Sigma Di466) which are cheaper but you'd need a remote flash cable that goes from your hot shoe to your flash and isn't very long, these are good for off-camera flash but obviously shooting with the camera with one hand and the flash in the other or on a bracket etc..

So basically, for strobing you'll want a manually adjustable flash (430ex, viv 285, Nikon sb-24 and newer) but for just a more powerful flash to stay on camera a cheaper full-TTL unit will do.

Strobing was where I was headed.. cool well you've answered my question

Worth buying second hand or new?

Sure, as long as they're in good shape. Good models for strobing are either the 430/580 canons and earlier models or the Nikon SB-24 and newer (25, 26 etc..). Of course if you want to have one for strobing AND on camera go the same brand as your DSLR and make sure it's compatible before you purchase.

just save up and buy a 580exII i have one its the best you can buy from canon basicly unless ur after a macro flash

I've never done flash photography before and didn't think running out and buying the best would be a smart move

will be cheaper in the long run y buy one and upgrade later y not get a good 1 from the get go

cant go wrong with sb's. they are all you need.

and if you shoot Nikon, then they're compatible too. I've got a Canon 40D but I've also got a Nikon SB-25 BUT I also have a 580II. The SB is great though, can't fault it.

Is there any downside to using the SB over the canon?

Yeah man, live I've said ten times now, if you own a canon DSLR a Nikon SB flash will NOT be compatible in the cameras shoe and then swap what i said for nikon camera & canon flash.