The Samsung Galaxy S 4G is an evolutionary upgrade to the Vibrant with a larger 1,650 mAh battery, a new battery cover, Android 2.2 (Froyo), HSPA+ 21 Mbps support, and a front-facing camera.  The form factor is exactly the same, and save for an updated version of Android, the software interface is almost identical as well.  It launches on Feburary 23rd (Wednesday) for $199.99 after rebate and a two-year agreement.  I spent the weekend toying with Samsung's newest Android handset, and here's what I discovered:

• Android 2.2 is a nice bump from 2.1, though with Gingerbread appearing on handsets, Froyo is so yesterday.  Joking aside, the inclusion of 2.2 brings the usual improvements - app storage on the SD card, Flash 10.1 in the browser, the ability to use cloud-based programs like Google's "Chrome to Phone," and more.  Given Samsung's track record when it comes to software upgrades, I'm curious as to whether the Galaxy S 4G will ever see any updates past Froyo.  If you're okay with that (or you're purchasing for someone that won't care), it's a non-issue.
• The Galaxy S 4G ships with Inception instead of Avatar.  Leo DiCaprio instead of blue alien people.  Funky dream world instead of mythical planet in another galaxy (pun intended!).  Sounds like a fair trade to me.

• Interestingly enough, I didn't notice a speed bump from Android 2.1 on the Vibrant to Android 2.2 on the Galaxy S 4G.  The 1 GHz Hummingbird processor works well enough, but the Galaxy S 4G is plagued by the usual hangups that the Vibrant suffers from.  There are some interesting quirks with the capacitive buttons, though.  Every now and then when the device is taxed, the buttons will simply freeze for 3-5 seconds; pushing them does nothing.  After a few moments, the phone jumps out of whatever temporary paralysis it was in and goes back to work.  I have no idea why it does this and if it's my review unit, but be sure to let me know if it happens to yours.
• The front-facing camera is a nice addition, and T-Mobile bundles Qik out of the box for video calling on the fly.  Wi-Fi calling is a nice touch as well for those times where you're outside of T-Mobile's coverage area.
• Like the Vibrant, call quality was good, with a loud earpiece and strong speakerphone.  I took the Galaxy S 4G to a T-Mobile dead zone over the weekend, and held the connection, though the audio was too choppy to carry a conversation.

• While the Galaxy S 4G isn't the first HSPA+ device to land on T-Mobile, it is the first to offer "theoretical peak download speeds of up to 21 Mbps."  As you would expect, I haven't seen anything near that in real life testing, but speeds are decent nonetheless.  I've regularly received download speeds between 5 and 6 Mbps, with one test going as high as 8.2 Mbps on the download.  Uploads range have been less predictable, ranging from 0.5 Mbps to 1.9 Mbps, depending on the location.
• Samsung has boosted the battery from 1,500 mAh to 1,650 mAh.  While it may not seem like a lot, the extra juice made a difference in day-to-day use.  With moderate use like calling, text messaging, browsing the web, and e-mailing, I was able to make it into the second day before the phone powered down.  It's not going to keep the die-hard media fan happy, but if you're coming from another Android device with a smaller battery, you'll probably notice the difference.

The Galaxy S 4G is a good revision from the Samsung Vibrant and can easily compete with the T-Mobile myTouch 4G and G2, but given that they're all in the $199.99 price range, I'd probably recommend one of the latter two due to the speed differences.  What speed differences, you say?  You'll have to wait for the full review!