Once an obscure workshop tool, the biscuit jointer has gained popularity greatly over recent years, and a good selection of tools are now available.
Biscuit jointing is a very quick and easy way to join sections of timber together. It is ideal for situations where a number of planks need to be joined side by side to make a wide board (e.g. for a table top), or for when you need to help fix and align parts of a project as you build it. It is a quick way to achieve many joints that traditionally may have been achieved with dowels, or loose tenons or laths inserted into routed slots. It also allows very good alignment of mating surfaces - even if the pieces of timber to be joined are not uniform in thickness, while allowing a small amount of latteral movement between the surfaces for final positioning.
Biscuits are small lozenge shaped wooden tabs made typically from a laminate of compressed beech wood. You cut matching pockets into the side of the items to be joined with the biscuit jointer, and then insert the biscuits into the pockets with a little glue and assemble. The glue causes the biscuits to expand in their slots helping lock the whole assembly together.
A biscuit inserted every 12" or so will make a strong well aligned joint, and cutting a biscuit slot every ten seconds or so is easily achievable - so the whole process is very fast.
The biscuit jointer uses a small thick circular saw type blade mounted in a mechanism that allows it to be safely plunged into a work piece to a predefined depth, and with great accuracy. The pocket it produces should exactly match the profile of the biscuit (a depth stop allows you to select the correct amount of plunge for the size of biscuit you are using - there are three common sizes). The jointers use a fence to control placement of the cut with respect to the edge of the work and to keep the cut square and parallel to the edge. Most have a fence which is adjustable to allow the amount of offset to be controlled (this is a highly desireable feature), and some also allow the plunge to occur at an oblique angle which is useful for stregthening joints that do not meet perpendicular to the surface they are fixing to, or when sinking a biscuit pocket into a piece of timber with beveled edge.
In a word: accuracy. The accuracy of biscuit jointing is largely dependent on the quality of the fence. Alas this is one item that can be a bit of a let down on some of the cheaper models. Not only does it need to be easy to adjust accurately, it needs to remain parallel to the plane of the blade at all times - even a tiny misalignment in the fence will very soon be multiplied into bigger errors the further you get from the biscuit location.
A smooth plunging mechanism will aid working. Typically you will simply push the body of the jointer toward the work to make your cut. The spring of the plunge mechanism will return it to the "unplunged" position when you release the pressure. When working on thinner sections of wood (i.e. trims etc) Ideally the handles of the jointer should be designed so that you can assist the plunge operation a little by compressing them so that you don't need to push as hard against the work piece.
A good power switch that is easy to find and use will allow you to concentrate on the jointing task.
A fence that can be flipped up out of the way is handy since it will allow pockets to be sunk away from the edge of a piece of wood.
Slow start is nice as it eliminates kick when the machine starts.
Dust extraction is often provided by a small collection bag on the side of the machine. On some, these actually work quite well (the dust produced is reasonably coarse and hence not as hazardous as that produced from many tools - although it is still wise to wear a mask when working on materials like MDF). With dust collection in place these tools create surprisingly little mess.
It is now possible to get some cabinet fixings (hinges etc) with biscuit tabs built in. This makes adding hinges to doors etc. very quick and easy. There are also biscuits made from alternative materials that are suitable for joints in materials like Corian®.
It is possible to cut biscuit joints with a router and a suitable biscuit jointing cutter. However it dispenses with many of the advantages of the dedicated tool, and is much less satisfactory than you may at first imagine.
It is a much slower and less intuitive task when used freehand - it is also very easy to damage the work piece if extreme care is not taken - the tool is not plunged into the work piece, but instead must be positioned on it without allowing the blade to come into contact, and then slid into the work piece to start the cut. Needless to say this is a routing task much better suited to a router mounted in a table.
Since the diameter of the router cutter is much smaller than a real jointing blade, you can not get the required pocket with a single plunge either; the router will need to be slid laterally along the slot axis to widen the slot to a suitable width.
Finally changing the plunge depth usually requires changing the size of the bearing on the end of the cutter - again not a quick process.
Before buying it is always worth reading the current reviews since it is not unknown for some of the higher end brands to have produced jointers that are not well regarded, or for some of the budget models to perform well above expectations.
Since jointers are a relatively new phenomenon, buying second hand is not as easy as it is with some tools. Assuming you find a second hand deal at all, then use the "What should I look For?" guide above to decide.