I've got a ~$190 661 turtle suit that's designed to work with a neck brace, and a Leatt Adventure.
They work together fine, but the jersey becomes a real PITA. Basically, if the jersey's head opening would fit over the top of the neck brace, it would be fine - but it doesn't.
So you've got to put the armour on (but don't zip it up all the way), then the jersey, and then get someone to help feed the Leatt into the jersey and armour. Then reach under the jersey and zip the armour up properly.
There's no way you'll want to take it off between races!
I've seen some blokes put their jersey on last, and leave the back part of the jersey on top of the neck brace. I tried it, and it was frikken uncomfortable - pulled the whole lot back on my shoulders.
I also find that the bottom of the back protector always comes out of my nylons. Doesn't really matter, but is bloody annoying in the pits.
The velco shoulder straps on the turtle suit are useless too - they're supposed to hold the shoulder guard part in place, but just come undone all the time instead. I don't find that the should guards move much, but maybe they would in a crash?
I'm not going to replace it, because it basically works fine - but I'd look around a lot more before buying again...