I train alone currently because no one else can work out with me in the morning....but I prefer to have an experienced training partner usually. With a good partner, you constantly have someone to hold you accountable and help you push through limitations when you would normally pull back if you were by yourself. A good TP makes sure that you're always pushing yourself harder and not wussying out. They can also help you make corrections on form and technique.
Oh, but one VERY IMPORTANT thing......a good TP knows how to STFU!!! If there's one thing I can't stand, it's fools in the gym who would rather waste their time choppin it up rather than training hard. The gym is for movin weight, not your mouth.