I've tried swype and shapewriter quite a bit. SlideIT i have not tried as it just came out and I am kind of annoyed with all the stupid spamming they or whoever it is, has been doing, even before it was released.
Note: most of my swype experience is with the leaked copy. I have only used the beta a little bit.
Swype seems a little easier for sloppy people. Or if you kinda forget how to spell and mis-slide a bit and then correct yourself it usually can figure it out. I don't know what lexicons they start out with but I think swype might have a larger one of common words to start, which makes it appear more accurate for first impressions / initial use. I didn't like backspacing to delete stuff with swype, it's sooo slow and I really don't enjoy doing the little swirlies to get double letters. works most of the time. Keys are big, can be a plus or a minus.....
overall Good software, I hope with this beta it means they are changing their tune from the backlash of people upset over their oem only approach. no reason they can't do both. or at least let carriers push it out to older phones if they want.
Shapewriter initially when the leak of swype first came out wasn't so hot. It had accuracy issues and some common errors always seem to pop up like four instead of for ...etc. It does have smaller keys and for some that's a turn off, personally I like the keys better since you have less chance of sliding through the wrong keys.
Since then, there have been several updates to shapewriter and it's free on the market. I don't know if this is a temporary promotion to get a user base going, or if they will start charging again, but for now it's free and I'm happy to use it. Initially shapewriter will seem a bit less accurate vs swype. I emailed someone at the company and they told me they use a very minimal lexicon and prefer to keep a minimum set of words if possible to improve system function. Which makes sense. You can have a huge dictionary with every word possible but if you don't use 80% of them they just take up memory and slow the recognition down.
I have found especially with the updated versions that after a few days of use and letting it add in some of my words that it's accuracy is just as good if not better than swype. I'm not sure, but I think the program starts to recognize how you slide? I don't know if that's true or not, but it does seem like it gets more lenient with your sliding the more you use it. I also like the nice quick tap to hit the number pad as well as the cool commands you can do like sliding from command a to l which will highlight everything making deleting a selection basically 1 button delete :icon_ banana:.
Theres also slide command copy / paste / cut / lit (literal mode, ie no spaces auto caps... good for addresses and passwords things like that).
Only downside I really see now is that some characters like _ are a bit hard to find in shapewriter. I think you have to use the @ sign and then use the word prediction box to slide over to find it. As a positive they seem to have quite a bit of support for multiple languages and you can change the language by hitting one key I believe.
Overall? I think swype wins initially and depending on how oems intergrate with the hardware on new phones and theming swype might be the top. However, at least for me and as it stands now, if you give it a few days time and let the program learn the words you use, I think shapewriter has enough little "cool" things going for it that it's the better of the two. I really like not having to slide or swype double letters. nifty once you get used to it. I think if people give it a few days or a week the program really starts to shine. It did for me and I really appreciated the fact that they supported android by selling to us users through the market and now it's even free, while swype..... well as far as we know right now, you will not be able to use it without getting a new phone....or someone rips the program and pirates it out to roms...:icon_ devil: They seem to have a spam problem on their forums last I looked, but if you email them, they seem open to feedback, criticism and all and I like they have really worked and gotten quite a few updates and revisions out.
SlideIT I have no clue, I think winmo people like it maybe? I haven't heard much good on the android front yet.
anyways, this is getting a bit long. Use what you are happy with, swype and shapewriter are both great programs and will work equally as well. Just I hope people give shapewriter a little more use to break it in a bit before deciding to stick with swype.
just my opinion.