Many other people have done an excellent job of summarizing the plot, so I'll just cut to the chase and go straight to the review. Bleach was an excellent anime/manga through the SS arc. It had everything a great story should have—humor, action, drama, an excellent plot, and unforgettable characters. Just about every character in the series had a backstory and internal conflict, making each one an individual who stood out from the rest. The plot was just as impressive; it was compelling and original, filled with tons of clever plot twists and subtle foreshadowing. Not to mention it was brimming to the rim with an abundance of themes and excellent symbolism (you have to look closely, but it is there).
Notice the use of the word "was."
After the SS arc ended, Bleach began to spiral downhill. What made the series so compelling in the first place is now lost amidst nothing but pointless fighting, chapter after chapter. The plot, which was shaky enough to begin with at the start of the HM arc, has not advanced at all since then due to all of the monotonous battles. This is one area Bleach has truly stumbled in; what made Bleach such a unique Shounen series before was that the plot didn't exist for the sake of the battles, unlike many others in the genre. Rather, the battles were carefully constructed in order to advance the plot and lead to new developments in the story. Now, battles have little to do with the plot itself and exist solely for the sake of there being fighting. Characterization has also been pushed to the sidelines; many of the arrancar have little to no backstory whatsoever. The few that do are overshadowed by the endless supply of minor characters Kubo keeps throwing in. This also goes for all of the Soul Reapers; most of the original characters were extremely complex and dynamic, changing throughout the course of the story (such as Rukia and Byakuya). Now, they've become as static as the arrancar they're fighting, which sucks out the last redeeming quality of the series. The end result? A story with poor characters, no plot, and an endless supply of pointless battles that kill the series. My advice to anyone wanting to watch the series is to stop after the SS Arc; though it does end on an apparent cliffhanger, the arc itself is standalone and deserves to be watched separately from the other arcs, which only serve to drag it down.