Like noted before: it depends on your goals and personal situation.

I had a little under 15K miles stranded in Mileage Plus. I bought something through the UAL website to get my balance over the 15K threshold and transfered the 15K miles to 30K HHonor points (transfers can only be done in multiples of 5K). Great deal for me: the points are helping me a lot to get 6 free nights in Hawaii, in MP I had no use for them at all.

These days I only use my MP account to earn miles with IDine: this earns me 10K HHonor points every couple of months.