Flash is a very useful tool, but like any other tool it's only truly effective if it's used correctly and in the proper context. So when you're setting out to design a website in Flash, ask yourself the following questions first.
What would I be using Flash to accomplish?
Really, what you should ask yourself is "what's the point?" If you can't answer that readily, then maybe you shouldn't be designing your web page around Flash. You might say, "to make an animated user interface", "to display my animations", "to create a multimedia interactive experience for my users", or "to accomplish a graphical layout that would be impossible otherwise".
Those are good reasons to use Flash; "to make bright zingy lights and cool fade effects everywhere!", however, is only a good reason if you're just making your website for your own fun and not considering your end users. For most, the purpose of website design and presentation is to convey information and interact visually with an end-user. You have to consider whether or not Flash is really going to help you do that.
Will my audience even be able to view it?
Although Flash is becoming more popular and on newer computers sometime even comes pre-installed with the viewer as part of the browser package, that doesn't mean that every one who views your page has the viewer plug-in or will be willing to install it when prompted. It's also possible they may have the viewer, but not the latest version, and their version may not even support the version of Flash that your website or animation is displayed in. On average 80-85% of web users have some version of the Flash player plugin on their computer; while that's a good percentage overall, you have to also consider your target demographic and an increasing likelihood that that demographic will or will not have the player. If it seems more likely that they will not, you may want to consider not using Flash or, as many do, creating a pure-HTML mirror of your site for those who aren't Flash-capable (this is the best alternative).
Is there another way that I can do those things?
Image rollovers? Javascript. Glowing text? DHTML. Complicated graphical interfaces with positioned links? Nested tables and layers. Slideshow image galleries? These can also be done with Javascript.
The point being, many things that we often think of turning to Flash for can be done more simply through programming. I'm aware that not all web designers know these languages while you will more than likely be more familiar with Flash, but there are dozens of free sites (such as DynamicDrive.com or JavascriptKit.com) that provide pre-written copy-paste code for non-commercial users. Using code rather than Flash can save a great deal on overall file size for your web page; it can also ensure that your page will be more readily viewable without any plugins/software, as your average browser can parse and display most code without any additional plugins.
What about file size and download rates?
That's a tricky consideration. With so many using broadband connections now, it seems like it shouldn't be an issue; but even on broadband Flash websites with a large amount of content can take an extra minute to load and then display, and on dial-up that wait can be a nightmare. In the ever-growing "now" climate of instant-download web content, you have to ask yourself if you really think it's worth the wait for what you're doing. At some point you will have to accept that dial-up users are, in some cases, going to get left out in the cold while you move on with something optimized for broadband, but don't bog down your broadband users as well just to make things look flashy or pretty. If the extra file size doesn't serve an actual purpose, consider ditching it.
