I started in web development for a side job about four years ago, mostly using Perl. Then I moved to PHP & mySQL, still creating websites for small businesses as a side job. Once I graduated from college I started working as a full time ASP & ASP.Net developer for a magazine company. I've since quit that job and now do application programming for a living, but I still continue my PHP & mySQL side business going.
Here are some of the advantages:
1. It's a business that is easy to do in the comfort of your own home, whether you're doing it as an independent contractor or for a corporation.
2. There are lots of resources out there on each of the technologies - it's easy to find examples of what you want to do, since it's probably been done before.
3. The technologies you work with are geared towards developers of intermediate skill...It doesn't take a computer dork to learn PHP, HTML, ASP, CSS, etc. They're all simple languages that can be picked up in a few hours.
4. There is lots of web development work out there...Particularly if you're willing to learn JSP or ASP.Net. Jobs for ASP & PHP programmers aren't as common and don't pay as much.
5. Web development is fun...you get to be both creative and technical, and have a visually stimulating final product that you can show off to all your friends and family.
Here are some of the disadvantages:
1. Nowadays, everybody and their dogs can make websites, and everybody and their cats thinks they're good enough to do it for money. The market is flooded with developers with little real development talent, so you have to figure out how to set yourself apart from the amateurs...They can do it cheaper than you, so you have to prove you're better and the extra money is worth it.
2. Web Development combines two nearly mutually exclusive talents...Graphic design and computer programming. Lots of people are good at one and fair at the other, but I personally don't know anybody that is really good at both. To do it for a side gig, you have to be able to compensate for your inability either as a programmer or as a designer. Personally, If a client wants a super-visually pleasing site, I contract that work out to a graphic designer.
3. There is not a lot of stability in web development companies. Dot coms come and go like J-lo marriages.
Hope this helps - I'll probably think of more things later.
-SN