This endures as one of my favourite Christian books ever, because it is so well written and tackles a difficult topic boldly. I didn't find it easy to read, partly because it deals with more heavily theological issues than I am used to, and partly because it clashed somewhat with the views on salvation and grace that I had held and understood for many years.
I strongly recommend this for any Christian who feels up to the challenge. Depending on your background, you may or may not agree with all the consequences Packer discusses, but he remains very solidly scriptural throughout. You cannot go wrong by reading this.
Brave readers may also be interested to follow it up by reading RT Kendall's "Out of the Comfort Zone: Your God is Too Nice", which expands on many of the issues Packer raises (although be warned that Kendall is a lot less gentle than Packer!).