As a single episode, Mergers and Acquisitions isn't exactly a masterpiece. Taken in the context of the entire Season Four, though, it serves as a major tone-setter for the last shows of the season and offers some irresistible adult entertainment.
The latter comes in the form of Paulie Gualtieri, who finally gets to see his mother again after spending time behind bars. He is shocked to discover she isn't treated very well by fellow seniors and enlists some special help to take care of the mess. Meanwhile, Tony learns Ralph's new mistress shares his passion for horses, which soon turns into something more dangerous and leads to big trouble at home with Carmela.
Watching the Soprano spouses argue is always a pleasure, but this time the limelight is gleefully stolen by Tony Sirico, who obviously enjoys putting Paulie's more twisted side on display: his efforts to make Mrs. Gualtieri's stay at Grren Grove more comfortable are a great example of good grown-up comedy.
Actually, the Soprano family manages to reclaim the spotlight in one scene, but not in the way you might expect: I'll just say it involves Tony, his sister and a discussion about Ralphie's sexual quirks. Yikes.