A valve spring compressor will be a "must have" tool if you are going to do your own cylinder head work or just want to be able to take heads apart before you take them to the machine shop. There are several types available. The one you choose will depend mainly on the type of heads you plan to work on.
Universal Manual "C" Clamp Style
This is the most common type of valve spring compressor. It is shaped like a large "C" and has manually operated lever to remove the spring. It works fine for most stock type automotive cylinder heads. Its main limitation is the size and pressure of the valve springs you are using. There are adapters available that enable it to be used on newer overhead cam engines as well.
Air Operated "C" Clamp Style
This type of compressor is basically the same as the manual one except it has an air operated cylinder that compresses the valve spring. If you plan to be disassembling and assembling heads for a living, this is the type to get. This type of spring compressor is also available in a heavy duty version for removing high performance valve springs.
The air powered type has two buttons you use to compress and release the spring. With this tool you can generally take a V-8 cylinder head apart in less than a couple of minutes.
Stud Mounted Valve Spring Compressor
The stud mounted tool is mainly designed for use when the cylinder head is still mounted to the engine but can also be used on the workbench. It can be used to change valve springs with the engine in the car. This is mainly used on racing and high performance engines that require frequent valve spring replacement. It can be used on the work bench but requires that the cylinder head be bolted down to keep it from moving.
This tool is also handy when breaking in camshafts with only the outer spring of a double type spring. Once the break in is complete, the inner spring can easily be installed on the car.
On the car valve spring compressors
There is one type of valve spring compressor made to remove springs with the engine still in the car. It works "ok" but I would only recommend it for occasional use as it can be very time consuming.
This tool really does not work very well on large double and triple performance type springs. It does do an adequate job on stock valve springs.
Other Types of Valve Spring Compressors
There are several other types of valve spring removal tools.
One type is made for use on heads that have shaft mounted rocker arms such as those made by Jesel.
There is a "c" clamp type that uses an actual screw handle to compress the spring.
There are also several "vehicle specific" tools made for use with late model overhead camshaft cylinder heads. These are generally dealership type tools.