Quite. On ATW it seems to work well. That may be helped by the nature of the routes like south Wales to Manchester/north Wales, where there a lot of smaller passenger flows against very few who are traveling the whole way.
I think it only really works for on-board staff, as not providing reservations on any web sites or via telesales means that most passengers don't bother trying to jump through the hoops needed to reserve a seat in the first place.
I can't speak for Chiltern, as I've only ever used them once, but by the sound of it it works well there too.
I don't think I've ever used Chiltern, but their web site says they only offer reservations on a few peak trains, and then only in their version of first class. Travelling without a reservation is fine if you're travelling alone, and would have no problem standing if your train is busier than usual. It's also fine if Chiltern always use trains which are longer than they need to be, although there are other TOCs which could do with the stock if so.
Having passengers wandering through a few carriages looking for a suitable seat would, to me, be a symptom of a system which doesn't work well. I don't know if that happens much on Chiltern, though.
I feel the Crosscountry way works better, especially as they definitely don't use trains which are longer than they need to be.
I was travelling on a fairly crowded Crosscountry train a while ago when a group of around a dozen people got on at Leamington. Most of them sat in their reserved seats along one side of the train. A couple of them found that someone was sitting in their seats, and decided to stand rather than ask them to move or look around for a free seat. The end result was that they ended up travelling together, and in something close to the intended level of comfort. I don't know what they should have done had they boarded a London Midland or Chiltern train instead.
While it cost Crosscountry some money to adapt trains for electronic reservation displays, a situation where reservations are strongly recommended, but are easy to make (you don't even need to prove you have a ticket) appears to me to work better than anything else I've experienced. And no question of a £10 charge to change your reserved seat.