On one hand, I agree with Gladjo and JohnTan that sometimes finding out is more of a headache than not knowing - esp if one has children - perhaps the best strategy is to invest more in the relationship and for those who has chosen to do that, kudos to you.
On the other hand, who can put a price on peace of mind? It was extremely painful but great to know. For me, it was worth every cent. Even after I knew, there are even now moments of fear, bewilderment (when the ex is not showing remorse and moreover starting attacking you like you are an enemy), yes, the skeletons in the closet came out, the can of worms is opened, the dirty laundry is washed in public and don't get me started on lawyers. Guess I am just that person who wants more closet space, to feed worms to the pigs and think anyway the expression "hanging dirty laundry in public" was from Napoleon who was an adulterer so who cares what he thinks. Don't let something that is not your fault make you feel ashamed.
Since finding out though, my world has plunged into something akin to the world that was in the Matrix movie. I took the bloody red pill. The world is a darker place when we take off that rose tinted contact lenses. People came out to me from every corner to tell me about their experiences on adultery, whether they were the betrayed or the other women or the betrayer. People are truely very messed up by adultery, including myself right now, not judging...
I started seeing people in shades of whether they were sociopathic, apathetic or empathetic... Negative: I used to love a lot of people very much, but I've become more wary of certain people. Eg, people who want me to feel responsible about my husband's adultery. These are not your friends unless you are a masochist. Positive: I used to hide things from my ex, because he was abusive and everything I do will be scrutinized for criticism (he was nicer during the time he was secretly committing adultery, but that was not the real him anyway) but now I can be more honest and open in my life. Did staying with him make me a masochist? Yes, but I have retired from that role. Not into M&S. Negative: Legally, it is horrific to go through a divorce with a sociopath even if one has iron clad evidence. Positive: I get to retain my sanity and have a new life without a sociopath. Negative: Statistically, men gain 10% more $ and women lose 27% $ during a divorce. I felt it. Weigh that in when you are considering a divorce. It also means that men will pay 10% for their wives to stay in the marriage.... I can understand that because we are truely very good to have around.

Positive: I prefer to lose that money than to stay in the marriage...hahhaha!
Some of you have decisions that you have to make, and you are going back and forth, back and forth. You have my deepest sympathy... it is really not easy.
K, how does a good PI help? First, whenever he revealed to me scary news, he also offered solutions and alternatives, the band aid whenever there is a cut. Also, very experienced PI can advise on man's behaviour and thoughts, and can see whether there is a way for the relationship to work. Also does not take advantage of the broken-hearted women when they are vulnerable. Really at your darkest hours, you need that professional who can say a word to make things lighter so you do not sink so deep into the mire, and yet gives you solutions. I'm really into that balance of sensitivity and solutions. Oh! Forgot to mention - he was great at giving step-by-step instructions on what to do in stressful situations.
Back to the decision of not getting a PI and preferring to invest that emotions and money into the family - there is a book that my friend who was cheated on twice recommended, it is a Christian book called 40 Days of Love. Apparently it worked for her, she used the day to day advice surreptitiously and her husband told her he never felt so loved in his life. I think it offers a tool for people with children and if the husband is not abusive.
So... which pill will you take? Think about it before engaging the PI....