I have the captivate which is a galaxy s phone, (almost same as the vibrant) so just thought I would try to share my experience with it. I have no experience with the x and am sure it is a nice phone, but cannot speak to it personally. The XDA developers made a program which makes the captivate SUPER easy to root so kudos to them. After rooting, it was also super easy to do the "one click lag fix" which is now just an app you download and run. It was also super easy to run the 1.2 ghz overclock from XDA in Odin. After running both the lag fix and the overclock, the phone is running 2500ish quadrant scores. The highest score I ran was a 2693. I have a screenshot if anyone wants to see it, but had a problem trying to post it in the Captivate forum. The phone is really really fast. I can just blow right through apps in market place where as before I had a few seconds waiting for market apps to load. The "voice to text" speech recognition comes back almost instantly where there was a few seconds wait before. I "raced" an iphone4 here at work with one of my friends and after we both cleared our caches, and history and cookies to make it fair, opening really large pages, huffington post, etc, the captivate was opening the pages maybe, 4-5 seconds quicker. I don't want to say it was twice as fast, because it wasn't but it was definitely not close. The iphone did not open one page faster. Also apps just snap right open. The only issue I see with the overclocking is that when people hit the power button to take the phone out of sleep mode, it has "wake up" lag which is another second or two before the screen turns on. Not a big deal, but noteworthy and certainly worth the trade off in speed.
I had a blackberry bold 9700 previous to this and the notification bar on the captivate is so easy to use it did not even occur to me to miss the blinking led on the blackberry. Phone takes great pictures, and quite frankly unless its pitch black out it really does take good pictures at night. I would have preferred a flash, but I compared night pics with flash from the blackberry to night pics with no flash on the captivate and they don't seem much different. It has a low light setting on it which seems to work ok. Takes really good daylight video, but I haven't tried any night video. Battery life is good, but not awesome. If you are on it full time without any breaks, not letting it sleep, browsing, etc, you will get about five or six hours use before needing to charge it, but that use is unrealistic. I am on it A LOT and have not gone home from work, (sometimes 12 hour days) and been worried that I was going to shut down from battery loss. One day I got down to red and just as an experiment, played a movie on the phone, and got almost through the whole movie before it finally shut off. Anyway, just my impressions, really like the phone, feels solid, just hope samsung is as solid backing it up, updates, etc. They seem to have to ball rolling so hopefully it works out.