There are very few areas in which the investment in a 3D tool like SolidWorks will not pay for itself many times over. A lot depends on what you do. The obvious advantage is that designing is easier and more accurate because it is done in 3D. Some of the less obvious advantages are:
1) Assembly drawings are no longer a multi-layer/multi-color/multi-linetype mishmashed twisted nightmare of overlapping lines that threaten to hypnotize those who are forced to try and make sense of them
2) Vendors such as machine shops, sheet metal shops and molders can use your CAD data directly. This ensures that your design intent is maintained and can reduce programming charges.
3) If you bid for work and must present proposals to the customer, a 3D rendered image looks infinitely more impressive than anything you will ever get out of ACAD.
You can also do animations if you pay the extra $.
4) BOM's are automatically produced from your assemblies. It is a challenge if you want to include a lot of extra info in the BOM such as cut lengths etc but if you just use a BOM as a list of parts then the built in tools will be sufficient.
5) Customization can be done via VBA, VB or VC++. You can do a lot of customization with only a basic understanding of VBA and a lot of cutting and pasting from the many examples that are available from SWX and on the web.