The inventory system in 1 was pointless. There was 1 weapon/armor at each tier that was better than all the others, so you looked for that one, and then a better version of that one. The ammo/armor mods were okay, but about half were still useless, so you picked the good ones there and upgraded them, the end. If you like clutter and garbage sorting, I guess it's okay. Also, the planet environments were NOT designed to be driven over... so geometry screws you over. And there's the same 2 or 3 building interiors and that 1 spaceship that get re-used for everything.
The weapons in 2 and 3 actually had different feels to them, so you were able to pick something that felt good to use and stick with it. I thought the hovertank in 2 was paper-thin and felt a bit too arcadey to be much fun. Fortunately, it's mostly optional.
The writing was decent in 1 and great in 2, fell lightly on its face in 3. Paragon/renegade choices in 1 were mostly accomplishing the same goal with noble/asshole dialog. 2 your choices felt more important, and the interrupt system was a good change. 3 is more of a straight saint/asshole choice, with asshole being outright destructive to your goals at times.
BUT!
The character writing is good throughout, and morality isn't one shifting point on a linear scale, so playing a more nuanced character is possible (I want to fight to the death for the innocent, but people who deserve no mercy get a boot to the head). Many of the larger choices are interesting and have far-reaching consequences through the games, from a characterization perspective.
As said, if you want RPG inventory management and slower-paced character (skills and abilities) development, 1 is it, the others less so. If you want to play a role and interact with characters who develop with/around yours while building a story your way, the series is good. I love the series as a whole, but it's not perfect. Your enjoyment depends on how much you overlook the not-perfect parts to enjoy the rest (and there's far more good than flawed, IMO).
TL;DR: 1 is a good game and worth playing. The others will make NO sense without it. If you play 1 and like the characters/universe/story play 2. If you like 2, 3 refines the gameplay well and provides closure. I highly recommend importing characters from game to game, as the concessions made from not doing so really hamper enjoyment in 2 and especially 3.