For a number of reasons (which I won't go into here), I decided to start having my coins graded by NGC. Generally speaking, I have been pretty happy with the results. EF and GEF usually get 62-64 and UNC have been getting 63-66. for the last batch of 15 coins that I sent in, seven of them got 64 or better. Overall, 71% of the post 1816 coins I sent in got MS62 or better. I expect that percentage to improve as I get better at the process and as I start sending in my nicer coins.
For pre-1816 coins, all but one got scores in the AU50-MS62 range. I also expect that scoring to improve as I start sending in my better coins. I held off on my best coins till I was more comfortable with the process.
NGC don't like scratches and they don't like rim nicks. So before spending money on grading a really honest personal appraisal of the coins you are about to send is worthwhile. A few friction marks are generally okay so long as they don't look like cleaning (e.g. hairlines). From experience, they are a bit more lenient with marks on older coins.
I have had a few disappointments but, when I look closely at the coin, I can usually see where they are coming from. The other big lesson for me is to be ultra careful in buying coins and especially from certain sources such as LC. I have had more disappointment with coins I bought at LC than any other source.