Playing Strength:
I'm a poor chess player myself, when I play against Slow Chess I usually play bullet time-control (2mins + 1sec/move) games against 2-ply, and score about even. I tested an old much weaker version on the ICC once, where it could take on IMs/FMs (Chess Masters) at fast time controls. The games were very fast with no increment though, and were decided on time quite a bit. The few recent results against strong human players I have suggest that in blitz Slow is quite good, although once in a while gets into a horrible position (blocked pawns, trapped pieces,) and might lose. The strongest human opponents almost always went into complicated/open/interesting positions instead of playing anti-computer chess; I've seen Slow play a lot of really poor games in blocked positions. It's hard to give an accurate guess of playing strength against computers, so I'll just say that from test games at Blitz time controls it's stronger than most free programs, but still quite a bit weaker than commercial programs, or the very best free programs.

I tested SlowChess 2.93 on the Win At Chess (wacnew.epd) test suite on an AMD Athlon 2700+ computer. This suite consists of 300 tactical problems that are usually easy for a computer. It can give an idea of a range of strength but isn't that meaningful. Here are the results:

Time in seconds: 1 2 11
Total # solved before time 297 298 299

I also tried SlowChess 2.93 on the ecmgcp.epd test suite. (On an AMD Athlon 2700+) This suite is much tougher, so it gives a better idea of the range of strength of a program. Again though, I want to point out that any test suite isn't that meaningful in predicting gameplay results.

Time in seconds: 1 2 3 4 5 6 7 8 9 10
Total # solved before time 74 98 111 121 129 137 141 146 150 150

Results File, ie. Slow's analysis for each position. (376Kb): ecm293.txt

Style:
I don't have the chess ability to really describe Slow Chess' style of play. Its evaluations do sometimes differ quite a bit from most other programs, but in general its positional bonuses are not extra large, so it usually will choose material advantages. (Sometimes a lot of positional bonuses are in effect for only one side, and can add up to enough to produce some pretty strange evaluations.) It doesn't try extra hard to attack the opponents king, but it you neglect king safety it has the ability to go for the kill. Also it has enough knowledge it should prefer bring pressure to the enemy king if there's nothing else to do. Just comparing Slow Chess to other strong computer programs, I suppose I'd say Slow plays a more tactical game than a positional one, even though it has trouble reaching decent search depths and often misses tactical shots. It will find some mates extremely fast though. Slow used to be an awful endgame player, but the latest versions have added endgame knowledge so Slow plays a reasonable endgame more often.