The SDS drill was introduced by Bosch a few years ago and it has revolutionised the ease with which one can drill hard masonry. For additional details on drill bits and usage see the Drill bits and Drilling FAQ.
The SDS drill is a drill with an enhanced hammer action that when compared to a conventional hammer drill is able to deliver hundreds of times the energy per hammer blow. To go with this it also has a different chuck design and special SDS drill bits to eliminate the possibility of bit slip, and also to withstand the force of its hammer action.
When drilling hard masonry or engineering bricks the difference is astonishing. Where an ordinary hammer drill may take minutes to make even a shallow hole, the SDS will pound through it in seconds. For this reason you also need to take it easy when drilling right through things like walls, because as you break through you can end up removing a large chunk of wall when the drill hammers its way out. If possible, always drill a small pilot hole through the wall first, and then drill inwards from both sides with the required drill size to avoid this.
SDS drills operate in 3 modes:
Like an ordinary drill, but maximum speed tends to be slower (under 1500 RPM) and torque higher.
The above mentioned enhanced hammer drill action. In spite of the extra performance, SDS drills also tend to be somewhat quieter than the conventional hammer drill.
Not all models have this (often referred to as "roto stop"). It greatly expands the range of tasks you can do. You fit special SDS chisel bits and use the drill like a mini concrete breaker. By selecting the type of chisel, the drill becomes ideal for chasing cable runs or socket cut outs in walls, removing individual bricks, removing tiles, and light demolition.
Safety clutch: Because the SDS chuck eliminates the possibility of the bit slipping, there is the problem of what happens if the bit should jam in the work. The mid range or better tools include a safety clutch that releases should this happen. Without a safety clutch you run a very real risk of being injured by the drill body, as it spins out of your grip. Broken wrists, or being thrown off a ladder are not unheard of results in these cases.
Sensible weight: i.e. 2kg not 5kg. Many of the budget tools are heavy. This is fine for demolition, but not so good for prolonged working.
Good speed control. Running slower results in a gentler hammer action. This makes starting cuts or marking out a cut much simpler.
Chisel position lock, for use in hammer mode: Many budget drills disengage the rotation of the bit, but leave it free to turn. You will not be able to chisel a nice straight chase in a wall with a freely rotating chisel. Better drills will lock the bit in one position, and the best will allow it to be locked in any user selected position.
Most SDS drills benefit from the occational application of grease to the shank of a bit prior to insertion. However some of the budget tools will often require frequent and more substatial lubrication by filling a grease resovoir with grease. After a while this will begin to ooze from various places and tend to get spattered about, which is not so good for keeping either the work piece or the operator clean!
SDS drill bits can be more expensive than conventional masonry bits (although usually last much longer).
You cannot put ordinary bits in an SDS chuck unless you fit a adapter chuck first. These typically extend the length of the drill further, and also do not allow use of the hammer mode. (Some SDS drills come with a replacement chuck for ordinary bits to circumvent this problem)
Even professional light-weight drills tend to be a few inches longer than a conventional hammer drill. This can make them more awkward to use in confined spaces.