Between Baldur's Gate II and Mass Effect 2 they were maybe my favorite developer. I even adored Jade Empire. Then suddenly, I lost all interest in them (some time after the EA deal happened - probably not a coincidence). I played Dragon Age: Origins and hated it. Played the first few minutes of Mass Effect 3, hated that instantly as well. Maybe they were a necessary evolutionary step for video games, obsolete now.
Many people have the sentiment that ME2 was their last good game, but really, ME2 did not fulfill its primary function. ME2 is great as a stand-alone game, but, in actuality, it is a disservice to the trilogy.
Mass Effect was announced to be a trilogy from the beginning. And ME1 was almost the perfect buildup, and its ending was good both as a stand-alone game and as a setup for sequels. Bar the technical in-game issues, it was a perfect setup for the future of the franchise. ME2 does not have these same qualities to continue the franchise.
ME1 ends with "the reapers are coming". That was a good ending as already mentioned. ME2? It also ends with "the reapers are coming", and that, within a trilogy, is simply a failure, because nothing of true value has been gained. The plot didn't progress. Mass Effect 2 should have moved from "the reapers are coming" to "the reapers are here", and ME3 should be primarily about defeating the reapers.
Instead, ME2 does nothing, and ME3 is forced to take care of both the arrival and the defeat. Of course it's going to feel rushed and crammed from the start, because one game has to pick up the slack of two. It's a miracle how good ME3 turned out, considering the mess that ME2 left behind.
I have to admit that I never played Baldur's Gate. I did play Jade Empire, but, Mass Effect was the first BioWare game I played. I did not find Mass Effect to be of lower quality than Jade Empire, so, I assume that BioWare was still good quality back then. I did play Dragon Age Origins as well, and while I initially didn't like it, in hindsight, I think it's better than Mass Effect 1 in some ways.
The decline of BioWare started with ME2, where basic coherency was no longer adhered to. It happens to coincide with the EA takeover, but I'm not sure we can fault EA.