Well, technically part three as well. I watched seasons two and three.
The series shifts a lot for season two, which was probably my favourite season. It covers the First World War, as well as the Spanish Influenza epidemic, so it naturally captured the history enthusiast in me. However, those two events also added a layer of intrigue and danger without it feeling at all forced by the writers. We know how the war will end, but we don't know how the characters will fare. This gives the writers plenty with which to work.
Unfortunately, I felt like in season three, with the stakes having naturally declined, the writers felt the need to manufacture levels of drama to match season two, rather than returning to the relative simplicity of season one. Season three becomes a bit of a bloodbath as a result, with a couple of huge characters departing. I still enjoyed it, but I did feel like the writers may be running out of ideas. How many times can the family almost-but-not-quite lose Downtown, or the service staff have questionable relationships? Bates and Anna held apart, O'Brien and Thomas scheming, Carson horrified by some tiny indecency, Mrs. Crawley sticking her nose where it doesn't belong, Edith alone; they're running out of ways to make it seem fresh.
The ending...well I'm not really sure what I think of the ending. My first thoughts were to really, vehemently dislike it. With respect to the series' tone, it does feel in a lot of ways like they're covering the same ground as they did midway through the season. However, it does add a huge level of complication to the story, and I think it may serve to move the Earl back into the spotlight a little more. I felt like he was increasingly pushed to the margins (an opinion which the character seemed to share) and while that made sense, I like the character and I want to see him have a little more prominence again.
It'll be a while before I see season 4; I don't think they've even started filming it yet, and I won't watch it until it finishes its live run and is released on blu-ray, so I have plenty of time to let my opinion on the ending settle. I just hope the writing team can come up with a fresh direction for season 4.