Just thought I'd add my $0.02 to this thread.
I've owned a Capri 22, a Capri 25, and now this S2 7.9. I've raced for the last 3 years in my Capri 25 with my direct competition being a J/24, J22, and the S2 7.9. The most formidable of which was the S2 7.9.
The S2 7.9 Grand slam was a racer cruiser part of a sub-line within S2 in a attempt to make racing boats that also served up some nice accomodations for cruising.
The S2 7.9 rates like a J/24 in speed, has 5'8" headroom for the highest point in the cabin, has a decent sized V birth, a foot pump sink, porta-pottie to starboard enclosed with a curtain, and came in 4 basic versions.. of 550ish hulls.
1) A fractional rig with a lifting keel with outboard
2) A fractional rig with a lifting keel with inboard (2 brands of inboard, yanmar and BMW)
3) A masthead rig with fixed keel and inboard (only 50ish of these made)
4) Exceedingly rare masthead rig with fixed keel, and outboard (these might have been converted number 3s).
The 12ft boom of the fractionally rigged lifting keel outboard model is what I own, and stepping up to this 26 footer from my 25 foot Capri 25, with fixed keel and outboard, the S2 is kind of a different animal. The J/24 and Capri 25 are similar boats in limited room down below, and similar speed to the S2 7.9, however, they really fit into a different category.
Ok, lets talk odd..
The transom hung rudder is, well, pretty massive. It's 60ish lbs of gudgeons (yep the gudgeons are on the rudder side) the pintles are mounted on the transom. This massive lifting body is powerful, but thick. It provides a sure amount of lift, and for sure drag on the boat. The tiller is not "heavy" in it's feel under sail, but the fact that it technically "kicks up" is probably an after-thought. One wants to make sure the pin is securely in place because the slightest amount of wobble in the kick can lead to a strong weather helm. Once properly placed and locked though the boat has a nice even feel, and following the tuning guides has a slight weather helm.
The standard backstay rig is a pinch style like the Capri 25. This provides adequate mast bend, but not through a huge amount of range. Most owners upgrade to a cascade style rig, and remove the wire and go to amsteel or dyneema cored line and a flicker. You see the standard mainsail has a significant roach, and a flicker is "helpful" to tack the main in light air especially.
I solo sail a lot, and wouldn't own a boat without a traveler at my fingertips while sailing, but that being the case the standard 5:1 of the mainsheet, I believe to be overkill in all but the higher (20+) wind speeds. dropping to a 4:1 allows one to keep the mainsheet at the attachment point and makes it easier to dump as the winds pipe up.
The lifting weighted daggerboard often turns some heads on these boats, and it certainly provides a boat with a flatter bottom, and in reality the daggerboard fully retracts into the boat. This makes for VERY easy trailer launching. Lifting the mast is still a job best for 2 or 3, but with some ingenuity it can be done alone. This was a primary consideration for me along with easy trailer launching.
The primaries are just a touch too far forward for solo sailing for me, and the cleats a bit a of a PITA even cross sheeting, but still better than most other boats its size for solo sailing. For crew it would be a non-issue, and actually are nicely located for the jib trimmer to sit far enough forward and high side to see the leading edge of the jib.
Most S2 7.9s are setup for racing. With the large roach main creating a generous sailplan and reducing your headsail inventory to basically a #1 and #3 (155, and 110) allows the boat to cover a pretty large wind range with few pieces of sailcloth aboard for inventory. In fact you can carry your #1 with a full crew of 5, well up and into 20 knots. With a reef point on the main, you can really scale up with the winds. The massive telephone pole of a mast isn't exactly "bendy" but it provides enough to allow enough flattening with some backstay pressure, also taking out the headstay sag.
Because of the fractional design (on most of the boats) the spinnaker is also fractional, making it slightly easier to handle than my old Capri 25 (a masthead rig). I've only sailed with the spin up a few times on this S2 but I can already tell it'll be much easier to deal with overall than the Capri 25. Given the transom hung rudder and barndoor size of it, I expect it'll be easier to keep the boat under the spin as the winds pick up too.
Beyond the good points if you are considering the S2 7.9... its spirited, but easy to control, launches easily, is big enough to handle some wind... Things to look for? Look for leaking chainplates, and soft core around chainplates.. Mast steps that are soft, or sagging. Soft core at the pintles (on the transom). Fat keel and or rudder (delam). Core issues are almost always due to neglect in bedding on hardware... Look especially around hardware that was "recently added" to the boat. This is a heavy layup boat, with a high quality end-grain balsa core. If the weave leaks it fights rot as good as any wood cored boat can... but problems must be addressed quickly to prevent a large repair bill. Many of the repairs can be affected from below (out of sight) and under the monkeyfuzz liner within.
The S2 7.9 is unique in that besides the 600lb daggerboard, the boat also has about 900 lbs of weight in the bilge area. This keeps the boat upright even with daggerboard up. It should be noted that the inboard models had 700lbs of weight added and true outboard models had 900... Several boats were converted from inboard design to outboard design and neglected this weight difference. Reasons for the conversion were multi-faceted, but generally boil down to, the outboard models are marginally faster (esp in lighter air) and the early BMW inboards are difficult to get parts for, and therefore its easier to pull the 1cyl diesel and just deal with the outboards.
These boats were made from 1981 to 1986 in a steady production run with limited production runs well into the 90s. About 1986 they changed the process with which the built the boats and it lends itself to less blistering issues versus the earlier models. The problems with blister are on par with other boats its age and aren't considered extreme.
These boats really do nicely upwind and can point with some of the best there is especially with the #3 up. The single upper and single lower with swept spreaders mean for a very stable rig that needs little help from a backstay except to help bend the rig.
Lastly if you need some true expert opinions on these (still growing in popularity) sailboats... checkout the national association at S2 7.9 Class Association