It's true - Heyer's mysteries are never quite as mysterious as they should be. I was fairly certain who had committed the crime by the second chapter, and my conclusion proved correct.
But never mind - I wasn't in for the mystery. The key is to not see it as anything other than a charming 1930s comedy of errors. I love Georgette Heyer because of her ability to write charming characters. I have read a fair number of regencies and mystery novels where the characters are flat stereotypes of the genre. Her characters, on the other hand, have a rare depth for the genres - they change, and our view of them changes along with the story. The main characters in "Behold, Here's Poison" are universally unlikable when they are first introduced, but over time I grew quite fond of them (in particular Randall, presented as a mean-spirited foppish sort at first, over the course of the novel he was revealed to have unexpected depths... still rather witty at the expense of others, but I appreciate that in a character.)
The romance did come a bit out of nowhere. I think this is the second of Heyer's novels I have read with cousin marrying as a plot point, which is perhaps a bit strange to modern eyes. This isn't her strongest work (none of the mysteries are really - read "The Grand Sophy" if that's what you're looking for. Sophie is one of the greatest characters in a genre populated by stereotypes.) It's enjoyable, and recommended for fans of the 1930s mysteries.