At his best, Larry Niven is second to none in the SF field. When he gets some good, original ideas, and takes the time to crank up a solid plot and a set of convincing characters, the result is something like "World of Ptavvs", "Protector", or the epic Ringworld tetralogy. In my view, those books are hard to fault in any way if you like hard SF.
"The Draco Tavern" is something different: a confection of relatively lightweight short stories, loosely tied in to a common theme. It is eventful, sometimes provocative, and always whimsically amusing. But it never threatens to grip your intellectual or dramatic interest in anything like the same way as Niven's novels. This is partly because of the flimsiness of the underlying premise. In the near future, the Earth is visited by a race of superhuman aliens, the Chirpsithra, whose vast starships periodically bring a menagerie of intelligent beings from faraway worlds. Rick Schumann, the narrator, is the owner and chief bartender of the Draco Tavern, a bar specifically designed as a meeting-place for aliens of all kinds. That implies the ability to provide all sorts of different environments, each with its own pressure, atmosphere, and radiation levels.
Presumably the whole scenario occurred to Niven in a visual flash: a bar in which all sorts of exotic alien lifeforms rub shoulders, rather like those in the Star Wars movies. A multi-talented human bartender flits from booth to booth, carrying regular cocktails, Irish coffee, and whatever weird potions suit each species. The Chirpsithra do not eat or drink in the Tavern, preferring to administer electric shocks to themselves with a device called a sparker. The hook lies in Rick's conversations with the Chirpsithra and others, in which he learns otherwise unknowable facts and techniques. How to build an intelligent computer, the real reason why stars go nova, whether there is life after death (and if so, for whom); and, on one especially memorable occasion, what the inhabitants of Earth were like when it had a reducing atmosphere. (Some Chirpsithra are really, really old).
These stories are great fun, but there is something fundamentally insubstantial about them. The whole idea of the Draco Tavern is unlikely; would intelligent aliens who live for millions of years really want to sit in a bar and drink, exactly like primitive, short-lived humans? What would the Chirpsithra get out of talking to us? Besides, Niven hardly scratches the surface of the key question his scenario raises: what effect would alien contact have on humans and their culture? Needless to say, the whole Draco Tavern world is radically incompatible with the Known Space universe with its protectors, Puppeteers, and kzinti.
Hence the four stars instead of five. This is a good book, but not a brilliant one. It will amuse and entertain, but the Chirpsithra not linger in the mind like Protectors, Puppeteers, kzinti, or Moties. Niven has not let himself down - this is just what he likes to do in between novels, and the short story format has very definite limits.