I havent read the other comments yet as Im short on time and have a computer install scheduled. But I can tell you that I had quite a few customers using ubuntu and decided to move them to Linux Mint instead.
Linux mint will be alot easier to move around in and is quite stable. Its built on the ubuntu base so it has all the bells and whistles that linux ubuntu has. However, It does not have the unity desktop which I think is pretty stupid. You can choose from quite a few desktop versions also.
Ubuntu (IMHO) is moving in directions that are not good. They are working with china, Screwing up the desktop and also spying on users for advertising purposes. This does not work in my book.
Try Mint instead. Its a very nice OS and even my computer illiterate girlfriend can use it and likes it. My customers also enjoy it alot.
Best Regards,
coffee
Addition: Clients that seem to do very well with linux are not interested in games as much as getting their email, Surfing the net, Working with documents and spreadsheets. My father is a lawyer and vacations now about twice a year in Flordia and Mexico. I took his Lenevo thinkpad and put linux mint on it. I would say he is computer challenged. But he emailed me from Mexico and couldnt talk enough about how he loved it!
Just size up your customer. If they have kids that want to play the latest, greatest games then its not going to be a good fit. However, For the customer that is tieird of viruses and stupid bloatware and just want to surf the net and get email its a great fit.
I also install teamviewer on their laptop. If they have a problem I offer remote support for only 35 bucks an hour. This is a small price and some will argue that its not enough. However, At that price point I bring in more revenue and customers do not look at the cost as excessive. Therefore, They will call more often when they have a problem.
Best programs to install for the customer -
1. Teamviewer
2. LibreOffice
3. Skype
4. Firefox
5. Warzone 2100 (gotta love that game

it can be quite adicting!! )
6. Thunderbird email