I got certified on Saba in 1984, and have about 25 trips there since then. It remains my favorite place to dive in the world - easily accessible from North America, pristine reefs, unique underwater topography. I dive exclusively with Sea Saba. Of the ~30 dive ops with which I've dived, they are peerless - they essentially invented the concept of "concierge diving" (at least, they're the first place I heard it from). They will arrange accommodations, nightly restaurant reservations, any specifics relating to diving itself, changes in travel plans. Most of their divemasters are there for years on end, and are experts not just in safety and on the reefs, but on the marine biology thereof. And speaking of the latter: Sea Saba was instrumental in establishing and continuing to foster the Saba Conservation Foundation and the Saba National Marine Park. If you dive with Sea Saba, you can actually learn about the pretty things you're seeing underwater.
As referenced above, Saba is quiet - not as quiet as it was in 1984, when there were all of two places to stay and maybe three restaurants on the entire island, but quiet nonetheless. There are no large hotels, only guesthouses, no nightlife, just some decent bars and restaurants. You go to Saba for diving, hiking the many nature trails, reading, and relaxing. Perfect R&R.
Accommodations: I've stayed at Juliana's and at Cottage Club. The latter has all its rooms in two story cottages, which are quite nice and contemporary, with some nice views. Juliana's has both some smaller and less-modern rooms, as well as some larger cottages, with kitchens and even outdoor showers; it's also a bit less isolated, because it's small bar and restaurant is a destination. They're both about equidistant from the center of Windwardside (well, Cottage Club is about five minutes further). I definitely suggest staying at one or the other, or perhaps renting a private cottage in Windwardside - there are more dining options within walking distance than in other parts of the island, as well as two well-stocked supermarkets.
On the diving itself, Saba has several things that make it unique in the area. Its reefs are not typical fringing reefs, but grow largely on volcanic rocks that were tossed into the water by early volcanic activity (some of which remains - there are some dive sites where you can stick your hand into warm black sulfurous sand!). There are several underwater pinnacles - more accurately, sea mounts that top off at 90-110', which are quite spectacular, and great for spotting pelagics, as well as bulbous resident groupers. All the sites are close to the dock - no more than a 10-15 minute boat ride, sometimes only 5 minutes away. The reefs are healthy, sea life diverse and abundant. The reefs on the windwardside of the island (which are often unreachable because of the seas) are wilder, with large fields of elkhorn coral and dramatic lighting from the sun bouncing off lava fields. Night dives can be spectacular - festivals of feeding nurse sharks, slipper lobsters, octopus, and cruising tarpon. Wild!
But Saba diving isn't for everybody. There's no shore diving, as there is in Bonaire and Cozumel; all the dives are scheduled. You can't roll out of bed onto the reef - a van is going to pick you up at a predetermined time for the trip from Windwardside to Fort Bay.
One final thought: Don't even think of diving on St. Maarten or anywhere else in the vicinity. There's nothing nearby that compares with Saba..
You can see some of my Saba photos here. Actually, these are from all over, but most of the blenny shots (and various others) are from Saba:
Underwater