I'm on their old data-only 12-months plan, $20 for 5GB+10GB data, 100+800 outgoing minutes and 100 SMS.
Pro 1: Cheap price
Current plans may seem more worth it (1GB for $1, for example) but to me, this is the best deal I can get because 15GB is way more than sufficient for my use. I rather save the $5 per month than spend it on data that I know I won't be using.
Pro 2: Loyalty is rewarded
Unlike other brands, M1 has some form of benefits if you're loyal. You can appoint up to 3 M1 numbers where you can make unlimited free outgoing calls to. This is great for families and couples who love talking on the phone. Some "Sun points" are awarded, which you can accumulate to offset your bills at some point of time.
Pro 3: Data Passport
Generally cheaper to pay and use data passport for first world countries. And it's a good-to-have option.
Pro 4: Cheapest supplementary plan
I've gotten my mom the supplementary plan, meaning she shares my data, call minutes and SMS. It's cheaper than what other telco offers and I know, people say that just a few more dollars, she can have a plan on her own. But hey, a dollar is a dollar. Over 12 months, that's $12, let alone some savings of $36 here.
Con 1: Disrupted network
The 4G network is generally smooth and good although there are some specks of weak data spots here and there, even when above ground. This is something I can never fathom how it can still happen when Singapore is so small and our data network should have been ironed out well even during 3G days.
Con 2: Loyalty is rewarded BUT could be better
It's good that they award "Sun points" but you'll probably also realise that they're almost non-existent. It takes forever to accumulate them into something useful so in a way, this is a useless benefit unless you're on an expensive plan or have the entire family using M1 plans under your name (i.e. on the same bill).