I’m not sure I recommend it; they have some annoying sync issues and I don’t see much development activity.
I have a lot of data in there (and automations set up to add even more data automatically); moving to a different service seems an “enormous” amount of work (and I’m not sure those have all the RTM integrations that I use/like).
I used RTM for years and years. Every couple of years, I take it for a spin and am always sad to see its lack of progress. My biggest example is the lack of notification badges on the Mac app (which is just a web wrapper.)
The closest modern equivalent is probably Todoist if you really loved RTM’s natural language processing.
I logged into RTM and was amused to watch a snapshot of my life in 2008. Toodledo.com is still around, and apparently it’s free (beware the business model!).
But I think the safest bet is to just use Apple Reminders, hard to beat that if you are into the Apple ecosystem.