I can only speak for myself, but I hate it because everyone seems to use it in different ways.
Say my budget is £250K and I do a RM search based on that. What I will find is some property that needs a lot of work and isn't worth anywhere near that much once the work needed has been costed in, but the place is empty and the vendor doesn't care how long it takes to sell and won't entertain sensible offers.
I will also find some houses that are worth 'around' that, and depending on the vendor's circumstances and the local market, I might be successful with an offer of 235K or I might end up having to pay, say, £265K.
And finally, I will find other houses that are lovely and probably worth more like £300, and the agent is marketing at o/o 250K so as to stimulate a bidding war, as a result of which the place ends up having 10 offers and selling for £350K.
The trouble is that unless you know the local market well, and perhaps even the individual house, it's very hard to tell which kind of house you're dealing with. Seems cheap? Is it a teaser or has it got something wrong with it? Seems expensive? Is it highly desirable or has great BTL potential, or is the seller a timewaster? It's very hard to tell, and a massive time-sink if you try to view all the houses that you think might be suitable for you, particularly if you'll be moving area and aren't very local. If you're really not very local at all, it can also be horrendously expensive. It's very dispiriting to travel a long way and incur overnight expenses to do a viewing at a house that transpires to have been a complete waste of your time because the level of interest puts it in a whole different price bracket from the one it was advertised at. It's rare for an agent to be honest about the fact that you might be wasting your time, and they never tell you the value of any other offers on the table (they aren't allowed to afaik).
But if you are the seller and have a highly desirable house, then I can see that the offers over system could work well for you, in the hands of a suitably proactive agent. It sounds like that might be you, @NewFriendlyLadybird, in which case good luck to you, but don't imagine you're saving people time by making it clear what amount you won't go below, because lots of other sellers use the system differently and it actually isn't as clear as you think it is. Depending on the general price bracket, £100K is potentially a very big undervaluation to use as the basis of your marketing and ime you'll probably be wasting just as many people's time as you'll be saving - sorry!
Just to add, the HR valuation is usually based on square footage etc and isn't always a very useful indicator of whether a house will have some desirability factor that isn't immediately apparent to a non local person, so while the HR is useful in many ways, ime that isn't one of them.
JMHO