I was traveling alone during the beginning of rainy season, so arranged for a tour guide to pick me up and do a private tour of 3 jar sites and the old city of Xieng Khouang. His initial price was $150, and I negotiated this down to $100.
Would've been an ok price for a private tour for one person in a town with not that many options, but my tour guide was horrible. First, after picking me up from airport, he offered to show me where he worked. He mentioned that he worked for a Japanese UXO aid agency and thought he was taking me there but actually took me to his guesthouse. He knew that I already reserved a hotel but was still trying to sell me on getting a room at his guesthouse. Note, I stayed at Maly hotel (review just posted) and his guesthouse doesn't have great reviews. He said that Maly was very far from town (it was only 15 min walk) and said I should've made hotel reservations through him.
The next day, during the tour found that yes, he spoke English but enough English only for tasteless jokes (saying that the 30K people killed during March 2011 Japan earthquake were fish food - and if fish were hungry, maybe there will be another earthquake and tsunami) but not enough English to answer questions beyond what I've already read in guidebooks (e.g. didn't know if there were any monks maintaining the bombed temple in Xieng Khoang). Note, the dirt roads to most of the sites were in pretty bad condition because of rainy season - saw 1,2 tourists ride bikes, but it's definitely a motorcross bike situation that you have to be used to driving.
Tour was completed at 3pm, so spent the afternoon exploring the market. This may not be for everyone (especially if you're only used to western standards markets) but I certainly enjoyed seeing all the different vegetables, fruits, meats and fish.
Took a mini-van to Luang Prabang the next day. Three tips - 1) get to station early so you can get a good seat by the window. Views are beautiful. 2) drink motion sickness medicine - 2 people in my ride got sick from the swervy mountain roads and even other Lao drivers reacted to the bumpiness. 3) if you're a girl, where a long skirt as not all toilet breaks will be done where there are toilets.
In all, happy to have seen the Plain of Jars, go to MAG and learn about UXOs, but if I were to return, would arrange a tour from Luang Pravang or VIentiane (saw many many options for both group and private tours at reasonable prices). Read in a few forums after the fact that are UNESCO certified tour guides but didn't see any of them when searching online or walking around town.