I've done London - Denpassar via KL with Malaysian. On the whole, the long haul sector is good. 34" seat pitch in economy is as good as you'll get anywhere and IFE is world class. I would echo WHBM's comments about quality of service. I do believe there has been a marked deterioration in recent years, and CC - while by no means bad - are less polite and attentive then they used to be. Meal and drinks service also gone downhill, although still nothing to complain about.
You'll find that the KL-Denpassar stretch is treated as a short haul flight, and on board service will be pretty much no-frills, even if the equipment is a wide bodied aircraft.
Given a choice, MH would still be my 1st choice airline to Bali.