Once a upon a time he was 2700, but blunders way too much recently. His greater achievement is that he is a renowned theoretician with the Black pieces, but probably knows this variation inside and out with White as well.

The Semi-Slav is actually a very exciting opening, but there are a lot of lines where you could lose just because you are not up-to-date with the latest theory (like the famous Botvinnik Variation). There are some lines that are totally unclear as well (like the main line Meran).

So if you really want to take up the Semi-Slav, then expect yourself to be memorizing quite a number of lines. If you can get access to chess books, try to get Peter Well's "The Complete Semi-Slav". Although some lines in that book were out of date, the book still demonstrated a lot of ideas and analysis for both Black and White.

after 1 d4 d5 2 c4 e6 3 Nc3 Nf6

In the Slav Black plays 4...dxc4 and develops the white-squared bishop outside the pawn chain; in the semi-Slav he plays e6 and keeps the bishop at home. Trying to develop the bishop while delaying dcx4 leads to problems.

Both are interesting and difficult systems. The Slav leaves it up to White whether to play 6 e3, which typically leads to a positional game with active play for both sides though Kasparov and now Topalov have shown how to attack with White in this line; or 6 Ne5 often leading to a tricky piece sacrifice. If white is committed to 6 e3 Black may have trouble playing for a win.

With the semi-Slav, it is slightly harder for White to avoid very sharp lines; the Botvinnik System, the Meran and the Shabalov-Shirov gambit are all sharp and both sides need to know what they are doing. Relying on instinct here is risky. There are quieter options for White, but they are somewhat less frequent.

There is also the option of 4...a6 and I think this is usually called a Slav though it might as well be called a semi-Slav.

The difference is that in the Slav (D11) black avoids playing e6 and waits for the opportunity to get in e7-e5, if possible, all at once. This usually doesn't occur. In the Semi-Slav, e6 is played early leading to the "Meran" or "Triangle" setup.

I play both lines almost exclusively as my main repitoire against d4 with black. I vary between them depending on how combative I'm feeling.

In my, admittedly limited, experience:

The main disadvantage of D43 semi-slav, is that the c8 bishop gets blocked in just as in the QGD. The advantage is that avoids the double-edged Botvinik variation in the Slav which, while even, is very tricky to negotiate for both sides. It also allows black to keep the center strong for much longer and gives the option of pushing either the c or e pawn to the 5th rank if white opts not to take on d5. If white plays Bg5 right away, then black can transpose into the Cambridge Springs defence of the QGD, which is a heck of a lot of fun and wins a lot of games at my 1500ish level due to the presence of a number of common traps.