I just finished a 2 week marathon playing the Paragon and Renegade paths in all 3 games, having bought the Trilogy pack 2 months ago and recently gaining a lot of free time.
I have to say this (some spoilers alert, for those who care):
The first game was incredible with atmosphere and scope. There was so much to do, so much to see, so many interesting people, and I loved the Mako sections, as boring as some of them were. However, the gameplay was clunkier than I thought it would be. The upgrades were tedious, I rarely utilized the abilities other than Barrier (and that was only when I felt bogged down in a fire fight), and I never used the cover system. I somehow didn't even notice it until I fought Beneziah. It all worked once I figured it out, so it didn't take away from the experience, but it took some time to get over the initial annoyance. Altogether, it was a great game with a few issues that could have been improved.
Mass Effect 2 was a completely different experience. The story was streamlined, the gameplay was altered, the upgrading system was heavily modified, but I felt it was all for the best. I went into this series for the individual characters and their stories, something Bioware has always done brilliantly, and I was not dissapointed. The only character I was disappointed with was Shepard himself. He went from the guy in the first game who was willing to make the tough decisions if I made him, but always came across as a man with the galaxy's best interests at heart, to either a glorified dick or the second coming of Jesus; there was no humanity. Everyone else, however, was interesting, varied, and I enjoyed just spending a hour walking around the ship and talking to them, from Jack and her childhood, to Miranda and her past, to Samara and her tough decision to kill one of her own daughters. They were all great people to just learn about.
The gameplay was altered in a way I enjoyed. I didn't have to worry about monitoring all of my equipment or switching gun and armor upgrades every 2 minutes depending on my adversaries, the cover system was definitely much more helpful to me personally, and abilities no longer felt tacked on or took a full minute to recharge. It was streamlined to feel smooth and still offer a challenge based on your own skill, not your ability to farm for XP and gear. The change suited my own preferences to how I like my shooters.
In the end, Mass Effect 2 was an amazing sequel. It improved where it needed to and streamlined where it needed to. I did feel let down by the fact that they replaced the Mako sections with that annoying scanning system, which took away from a lot of the scope of the first game, but the improved character interactions and the scale of variety of environments, along with the better gameplay, more than made up for it.
Mass Effect 3, despite what many people claim, is not what I consider "a massive letdown". Sure, it didn't really make any breakthrough changes to the story or gameplay and felt the same as Mass Effect 2, but I don't consider that a bad thing in any way. It tweaked the gameplay from the second game by adding the weight system which brought back the planning aspect from the first game, but didn't take away from action by having me switch upgrades to deal with new enemies on the fly. I had to plan my equipment before I left. Maybe I needed to focus on long-range combat, but keep myself varied if things broke out in close range and I needed keep my weight low so I could use my abilities when I needed them, so I'd pack an assault rifle with a scope and a SMG with the ultralight frame mod for those close situations, or maybe I'd be fighting in cramped quarters, so I'd grab my Graul Spike Thrower and a Arc Pistol, both with their respective melee attachments. It let me vary my play-style while still feeling simple and streamlined.
The story and character interaction did suffer slightly, in my opinion. One of the reasons I enjoyed the second game so much, as I said, was just the sheer amount of time you could spend with each character, just talking with them, doing their loyalty missions, and it just made them feel that much more fleshed out. The third game just seemingly glanced over a few of them (noticeably Garrus, in my opinion), and the returning characters didn't seem explained too well. So much happened off screen, and it all just seemed tossed aside with a few sentences for each. How has Thane been reconnecting with his son? How has Samara dealt with the grief of being bound by honor and code to kill her own daughter? It's slightly addressed, but not nearly as much as I'd like. Still, the scope of the war can be felt, and what is there is deep and emotional. My favorite additions are the little side-stories that you find on the Citadel as the game progresses, my favorites being the teenage girl talking to the turian officer and the crazy Asari huntress. You can gather how much this war has taken away from these people, even through such little interaction. I loved it. Mass Effect 3 delivers where it matters most and felt the most balanced out of the trilogy in regards to gameplay vs. story: neither took preference over the other to me.
Mass Effect 3 gets more hate than I think it deserves. My favorite will always be 2, but 3 was an amazing game in its own right. The endings left a lot to be desired (a LOT), but I've always been a firm believer in "It's not the destination, but the journey" and The Mass Effect series was one hell of a journey. I enjoyed every minute I spent with these games, and I'd do it again if I could, which is why I'm looking forward to Mass Effect 4. It probably won't match the awe of the first three, but I'll enjoy it anyway.