I have read every single Agatha Christie book published and must say that, though I tend to prefer Poirot over Miss Marple as a whole, 'Nemesis' is tied with 'Endless Night' as one of my two favourite books of hers.
Don't just take the word of those who say Ms Christie declined irredeemably in her final years - read the books and discover for yourself. Yes, her style changed toward greater characterisation and less fast-paced cryptic plotting, and quality-control wise she may have faltered once or twice, but times change and the world of the 20's and 30's moved on - so did her writing.
Expect a greater lead in to this story, as Ms Christie first reveals the crime itself, the characters relating to it then its positively chilling (and devastating) denouement. I challenge you, like Miss Marple herself, not to feel as much sorry for the murderer as you do for the victim.