If you own a mobile device with a touch sensitive screen, then plain and simple you should get a screen protector. If you don't you will probably regret it because even a stylus will leave scrape marks and scratches on the screen eventually. Even more so if a piece of dirt gets on the stylus to make it act like a diamond tipped engraving tool.
So how to know what to choose? A Google search will get you plenty of reviews of specific brands, but I'd like to point out a few things that may not be so obvious unless you've personally tried multiple types of screen protectors (like I have).
First of all, the basic idea is always to lay a slightly sticky transparent plastic sheet on top of your screen surface. It should not decrease the clarity or brightness of the display, and it should be pleasant to use the touch sensitive screen with your finger or stylus.
Some screen protectors are called washable, but I don't advise washing and reusing them because they will never be like pristine condition after they are used once. Even applying and removing multiple times can get dust on it, stretch it in places, and generally make it less than perfect.
Secondly, be sure to not destroy it when you apply it. You'll need to make sure the screen is very clean (I use a lens cleaning cloth or an eyeglasses cleaning wipe), and that you apply it from one side of the screen to the other. Preferably by pulling it only partly away from the backing paper and then laying it down and carefully smoothing it flat with a piece of hard paper or business card or credit card.
The idea is to make it lay straight and uniformly flat, and to keep bubbles out. Other than signs left by applying and removing and reapplying too much in the process due to improper alignment (some adjustments are okay if you catch it before you go too far), bubbles and dust are your main enemies. Keep smoothing out the bubbles and making sure the screen is clean before you start will give you much better success.
Does it have to be applied perfectly? No, not at all. And it probably never will be applied perfectly. But the better you get it on there, the less "defects" you will see. If you use it all the time, it's worth a little care to do it reasonably well.
What are the differences between screen protectors? Several main factors come into play. First is the hardness or softness. A hard protector will probably last longer, but it can tend to give you less "feel" when you use the touch screen. It seems to spread out the touch point a bit. One good hard protector I've used is from Boxwave. It lasts fairly long, is very clear, and applies well to the screen. I don't care for the "touchability" but it's not bad, and isn't a big deal with a Treo because you don't use graffiti on it, so there's really not that much stylus work anyway. They are a bit more expensive, but they last longer, so it probably works out not bad in the long run.
Soft protectors are sometimes too rubbery, and can tend to stretch when you push too hard when dragging the stylus across the screen. They also tend to collect bubbles more on the original application, and when you try to get the bubbles out you leave little stretches, and leave other small bubbles. Not a big deal, but noticable. The bargain protectors I bought from eBay really cheaply were this soft kind. I like them, but they may need to be replaced every few months or so.
The other factor you need to take into account is the affect on display quality. It should be very clear and transparent. A friend of mine had extras and gave me a few. I tried one and it left a grid sort of like a graph paper overlay on top of the screen. And even worse it ended up slightly off kilter so with that protector on, the screen always had this optical illusion of being very crooked! It was actually the best protector I've ever used except for that grid that ruined the screen. Both soft and hard protectors can be clear, so it's often just a matter of quality. You don't want one that introduces more glare (already a problem using pdas outdoors), or decreases the brightness. But that's something that you can only learn from experience with a particular brand or by reading reviews.
I just recently removed my "grid" screen protector and replaced it with my eBay cheapee. I had been considering separate cell phone and a T|X recently, but with the new screen protector I've rediscovered the beauty of the Treo display once again. It's like I've fallen in love with the device all over again. I can't tell you enough how much it's worth your while to make sure you are not stuck with an inferior protector. If you are, you definitely want to do a little research and find another option. Maybe some of our readers will share their experiences with various brands in this thread also.
One final important comment relative to the Treo screen in particular. It's the smallest screen of any Palm device. Why is that important? Because it means you have an option that is not available to the devices with big screens. You can pretty much take any screen protector for any device and cut it down to size and use it on your Treo. That means you can find some on sale really cheap, or even use protectors that you have left over from your previous pda.
It may or may not be worth the aggrevation of the cutting to save a few bucks. After doing that cutting myself a bunch of times now, I guess I'm going to keep cutting, but sometimes I think it might just be worth ordering them the right size!
But for now, I have a bunch of protectors left over, so I'm doing the cutting. The method I use is simply to lay the old one down on the new one and draw a line around the outside of it. Then I cut the new one to barely remove the line and it makes for a good fit. But I've ruined a couple also, so it doesn't always work.
The Treo came with a "universal" screen protector that was large enough to work for any size Palm device and guide lines for where to cut for each model. I suppose it might be helpful if I could make a copy at 1:1 ratio with now magnification and lay it on a protector. But I haven't really found that to be useful yet. I've also tried measuring the screen and using that to mark the new one before cutting. But I always run into trouble with the width of the mark. Do I cut outside or inside? Did I measure right? Etc.
I'd be interested to hear how other people have successfully cut them down, or if they just leave it with a flap where it hangs over.
So there you have it. Some thoughts on screen protectors. Time for you to add your own thoughts now.