I have used it on a few occasions. Reusability is indeed extremely good. I bought 2 sets from them.
For "flat" 2-dimensional castings, it works quite well.
For 3-dimensional castings, I tried making a few torsos for my Tau. Results were ok-ish, but it can be a bit challenging to align the two parts of the mould correctly. There is of course a mould line, not too bad depending on how good the two mould parts fit together, a cutter or a file can take care of it.
Devil is in the details, quite literally. It can be quite challenging to get them all correctly. In fact, it all depends on the pressure you apply and the degree of elasticity of the bluestuff: the colder it is, the lesser the degree of elasticity is and the worse the imprint. Hence you have a rather limited time to get the imprint done. Don't hesitate to start the mould over before casting if you feel it is not right, bluestuff is perfect for that.
I pour the water into a plastic tupperware for micro-wave oven (which I only use for that very purpose), warm it nearly to boiling point, plunge the bluestuff in it, maintain it under water with a cocktail stick (fingers work too but they tend to hate nearly-boiling water) for a couple of minutes, then make the imprint.
For 2-D castings, one imprint is of course sufficient.
For 3-D castings, wait until the first part of the mould is cold before repeating the operation for the second part of the mould (otherwise the two parts can have a tendency to slightly "fuse" together which creates a mess). Don't forget to make a few guiding points/holes so the two parts can be correctly aligned.
Apart from getting all the details out, aligning the two parts of the mould is where you'll also have trouble (depending on the quality of the guiding points/holes of course, mine were not so good).
Then, wait until the GS or milliput (or a mix) has completely cured. Don't try to have a quick look too early, it ruins the casting. So I tend to do it in the evening and leave it aside until the next day.
For some parts, it might be "easier" to find the appropriate bits from a bits shop (subject to price, availability, shipping costs).
Besides a few torsos, so far I have mostly used bluestuff to replicate some parts for my bases (imprints of terrain details, most long out of print).