If you don't pay for stuff it's extremely grindy, though there are some neat things and the combat is interesting, at least until you're tired of it. Things have changed a lot since I last played it, though.
I played it for ages. I did pay a little. I think maybe $20-$25 for about 8 months worth of gaming. Value for money if you ask me. The grind is far easier than other similar games. It really is possible to avoid spending anything but the leveling is limited because you need to open up slots for new frames and weapons. I spent my cash on Platinum, during sales only, and then used the plat for Warframe (character) slots, some weapon slots and an Orokin Reacter/Catalyst.
I just downloaded this game. Do enemies get ridiculously spongy in the late game?
Edit: If you're just starting out I can help you out w/ bro-tips(Boo-letz can ricochet off the Riot shield and kill" and spare booty. Hit up my steam. It's the same as my forum name and don't be afraid to hop on TS.