I think it's a decent game. If you haven't tried it before, it is at the very least worth a shot.
In saying that, I've had an account from between 6-7 years. Many changes have been made to this game since it came out, and I personally find it extremely boring now. That is not to say that this is caused by the changes, but a lack thereof. Make no mistake this game is 110% grind. When I got into this game I remember when I had my first squad mate playing Kiliken back when it was a defence mission in Venus. It was the first time I had seen the Rhino frame and it's abilities. To this day I think that the Rhino stomp is the coolest ability in the game. Stomping the ground so hard that for a brief duration all the enemies affected can't touch the floor. Very quickly my motivation for playing Warframe became a desire to build all the frames and test their abilities to their limits, quite literally only because the abilities looked cool(or so I thought at the time.)

But unfortunately outside of that, there's not much incentive to keep playing(for me). Aside from coming back to play the new updates for the main story line which amounts to at most only an hour of actual content and several more hours purely for grinding, this game is really easy to get burnt out on because there is a severe lack of variety in the things you can do in this game.
The problem with this game albeit it's a huge one is that unless you quit and play something else, there is nothing in this game that allows players to wind down the mental strain of endless grinding. This game is ONLY grinding. I don't claim to have a solution, but the problem is nonetheless easy to identify and I can only make suggestions henceforth.

For example; World of Warcraft is a huge grind, but the wind down is satisfaction from PvP and raids, and various sense of accomplishment through a competitive nature.
Fallout/Skyrim/GTA/- sure you grind through the campaign going back and forth across the map, but that's not the only option presented. You can almost do literally anything else.
I think that if Warframe had more an emphasis on PvP, free reign, or anything that incites a slight competitive atmosphere, this game might offer much more in terms of replay value. The only endgame you're really working towards isn't to be the best at anything. In the end it's all about showing off your aesthetics. So unless you're playing with friends or people you know, you realise it's all the more worthless. The game doesn't offer any sense of high difficulty/challenge reward. Long defence or survival missions regardless how long you labour, incites more tedious emotion rather than ecstasy.
But hey, I'm just burnt out on video games in general so I suppose that could've heavily influenced my opinions.

All in all, I do recommend this game for players to try. Maybe you'll reach a different conclusion after hours of gameplay and achieving your goals.

First initial impression 9/10
Current impression 7/10

EDIT 23/09/2020: Loving these new mechs and the fantasy-like infested open world.

EDIT 10/05/2024: I'M A WIZARD HARRYYYYY WOOOO