I've been using MLO for almost a year now. Before that, was using Toodledo. I switched because Toodledo ios client was lagging behind in development, and I was looking for an app that excelled on the iPad - which is my main device, but also on iPhone and PC. Right now, this is definitely the best option I've found for GTD on these platforms - although not perfect (will anything ever be?), I'll touch on that at the end. Where this is really good:
- easy to enter tasks as they arise into inbox wherever you are in the app (force-touch on iPhone as well)
- easy to process tasks in your inbox. When I process tasks, I add contexts, a flag, a star and then move to whichever folder I have for that "group" of items.
- all of these are configurable, here's how I use them:
Contexts: I use this for three things. Firstly, the usual
@context for the location that makes sense, eg. @home @work. Then a two letter code if the task relates to one of my team members, eg. HE or JF. Then a +number for estimated time, I use these: +15 min, +30 min, +1 hr, +2 hr.
Flags: Again you can define these, there are seven you can set up. I use this to highlight what I'd like to achieve This Week, Next Week or Today.
Star: If something is really important I will Star it. Useful for that urgent task that comes in. All of my views show Starred items at the top - so I can't ignore them

Folder: You can setup any needed folders to keep tasks together or as separate as you'd like to. For example, I have a Family folder, a Bills folder and a Household folder. I also have a Later folder, which will hide tasks that are moved to it - until I proactively review them.
You can make any task a folder, and can nest as many tasks as you need to as sub-tasks (there may be a limit here, but I haven't hit it yet). You can make any task or folder a project. You can also make any task or folder a goal (I haven't used the goals function yet).
You can make any task or folder "complete sub-tasks in order" which will then allow you to display only active actions.
Things I love about MLO:
Simple and easy to manage large lists of tasks.
The iPad app is really easy to move around. For example, if you have a view created to show today's tasks, but can't remember the context of a sub-task - if you change view to show all tasks the app doesn't lose the context - so it shows the same task in your full list of tasks, so you can easily understand where that task has come from.
Sync between iPad, iPhone & PC has been rock solid. You do have to wait for a few seconds when the PC app first loads, to let the sync complete, but the iPad and iPhone will push-sync.
Things I'd like improved:
Time needs to be better implemented. This is the same of all so-called GTD apps I've seen. There isn't a quick way to enter how long the task will take, and then sum those times in a filtered view. What I'm trying to do here is to see my today view, and see that I've picked 16 hours of tasks to do - so I'll need to better prioritise at the start of the day.
- I wanted to ask if Things 3 does this any better?
Let me know if you have any follow up questions. I've been overall very happy with MLO.
- Huw