Construction: 10/10 - Solid assembly with quality hardware.
Portability: 9/10 - Pretty darn small & light for a 15"bass cab.
Volume: 6/10 - That great portability comes at a heavy price.
Sound: 8/10 - A nice, refined, hi-fi (for a 15) sound at lower volume levels.
The Bag End S-15 D is about as simple and compact as a 15" bass speaker can be. At 18" wide & tall, the box is just big enough to contain the driver. This makes for great portability (Toyota drivers take note).
The box is very well-built, with heavy duty carpeting, rugged professional spring-loaded handles, thick insulation and a metal jack plate with parallel 1/4" jacks and a "banana" jack. (The "banana" is my favorite type of speaker connection because of it's high power handling and easy field maintenance, but I always appreciate the added flexibility of parallel 1/4" jacks.)
The speaker is a high quality Bag End driver that supposedly handles 400 watts at 8 ohms. The response is surprisingly flat, even through the midrange and up into the treble end of the spectrum. While it sounds much more impressive than many of the lesser 15" speakers I've tried, 400 watts might be a bit optimistic. Run alongside a 350 watt Eden 210 cabinet, the Bag End speaker eventually got fried, and had to be sent in for replacment. (Warranty service was nice and straightforward.)
The small cabinet is convenient, but unfortunately there's a big price to pay in terms of sound. At louder volumes, the S-15 D didn't have much low end to offer. The cabinet is rear ported. Placing it in a corner or near a wall can help in theory, but in practice, it doesn't help enough. This was my main speaker for a year and a half and I got consistent observations (and complaints) that the bottom just wasn't there. It got to be pretty frustrating, trying to use this cab as a stand-alone with loud drums and guitars.
For lower volume situations, the S-15 D works much better. With it's high fidelity response, high efficiency and mellow 15" speaker voice, it's a popular choice among the local jazz players with upright basses and low wattage amp heads. The S-15 D also seems to be more potent when paired with another Bag End, like the 15 with a coaxial tweeter, or it's 210 cousin. I recently saw a rig with the latter configuration, and it filled the room very nicely with the tight, punchy hi-fi sound Bag End is known for.
To sum up: The Bag End S-15 D is a very high quality product, but it needs to be bought and used in the right context to avoid dissapointment. Based on my experiences, I would reccomend the S-15 D for jazz gigs, or for use as part of a larger modular Bag End speaker system. I would not reccomend it (as some salesmen do) for high-volume use by itself, or run alongside a non-Bag End speaker.