It depends on what you are trying to achieve. If you want to lay out pipework and fittings in a building environment you would do a lot worse than look at Autodesk Building Systems. This product allows you to take architectural data and design, in 3D, pipework and electrical services.
If its the actual design and manufacture of the fittings that you are into then you need to look at a parametric 3D modeller. By far the most widely used in this industry is Pro/ENGINEER for which you can buy a pipe design extension (costs around 18000GBP with the extension or 4500GBP if you just want the core modeller) - but if you are looking for something dead easy to use and relatively cheap you could investigate Autodesk Inventor Professional which has a piping and fittings module (costs around 6000GBP)
Hope this helps ....