Difficult. Worth it.
This book was difficult to read. First, it was difficult in the sense that the language is older and at times I felt that I needed a translator to get through. Be prepared for a jolt If you're in the habit of reading contemporary works and then dive into this one.
Second, it was difficult in the sense of conviction. I found Bunyan's handling of this Biblical text to be superb and, as a result, I saw much of myself in both the Pharisee and the Publican. I felt convicted of sin, challenged in my prayers and understanding of God and relationship with Him, and encouraged to continue pursuing what began for me over 20 years ago.
Finally, if you're up for the challenge of non-contemporary English (and even if you're not), I would recommend this book to you. For a non-religious person, it would perhaps give some enlightenment into how Christians read and interpret Scripture, besides a primer on the Gospel itself. For a believer, I hope it would challenge you to see yourself before God in a new light, convicted anew over sin and cherishing once again the salvation you have graciously received. It's a worthy read for all.