They are relatively enforceable as part of a compromise agreement. However the last one I signed was quite restricted in that it was for 12 months and only listed 4 direct competitors. Normally the companies lawyer tries for more and your lawyer tells them to get real.
If you don't sign the agreement I suspect a lot of the time you'll just have to do gardening leave for the rest of your contract. Also don't forget if you were a director you still have a fiducary duty to the company once you've left.
What I'm slightly unclear about is how practical this is for sales folks where this is an expectation they will steal prospects or existing clients. I suspect you might only bother with lawyers if it looks like they are poaching from your existing clients.
In the UK non-competes in my view for engineering staff are basically unenforcable, IP theft is much more likely to succeed (and I've been involved in a direct copyright theft case).
24/7 IP ownership is enforcable but my experience is that most places don't care if you're not going to be a direct competitor. I always feel it's worth getting that in writing though, my current contract limits this to areas of the companies business. Our corporate lawyer limited this when we had to sign a new contract post a takeover (it was a bit broad and could have covered things like photographs of your kids). Also I have in the past asked if I'm contributing to open source as part of my work as like things to be clear.
IP rights is probably the one area I check in a new contract and usually it's fairly easy to get agreement for some minor changes.
Limited NC for v senior people for short periods possibly - and by v senior I mean board level.