I just made some beef in the potroast this weekend and it came out moist and delicious. It actually was almost soft like butter. It did a better job than even my pressure cooker.
If you are making a pot roast you may want to try to do the following. You may want to use the right cut of beef. I like to use prime round that I get from my butcher. They trim off all the fat and it's lean and tender.
I added the beef, along with a small can of tomato sauce, ground spices (whatever you like), freshly minced garlic, chicken or beef stock about 2 cups (the meat will release more liquid as it cooks), tiny bit of sugar, splash of vinegar and some whole spices in a cheese cloth (I like to tie a cinnamon stick, bay leaves, black peppers) and throw that in there.
I cooked mine overnight on the High setting (I started the crockpot around 11:30 PM and stopped the crock pot at 8 AM). The beef was perfectly cooked and almost fell apart. There was plenty of liquid and I poured it in a pan to reduce it further and add some frozen pearl onions and chopped potatoes and carrots to the stock as it was reducing. Poured it all back over the beef after the veggies were tender. Garnished it with parsley and all set. It turned out very good.