Just watched the whole thing in three weeks. Best show I've ever seen, no question. I was amazed at how the quality started high and stayed high. Most shows seem to have a 'feeling out' phase before they get good, but not this one.
I'm sure every opinion has been beaten to death but I'll give mine anyway. The show was extremely heavy on foreshadowing, so I don't understand why people have questioned the ending. Of course the machine gun and ricin were going to come into play - just like Salamanca's bell and dozens of other plot devices they hammered over and over. By the 3rd season I was having a lot of fun trying to predict how the foreshadowing was going to come into play. Sometimes I was right, sometimes they got me. Helluva three-week ride