First thing you have to realize is that there's limitless potential with your Omnia and I doubt you'll ever be 100% happy with it - you'll always be wanting to make it better and better! If you haven't done so already, you should flash to the "leaked" ROM and then to a custom ROM. A ROM, if you don't know, is what holds your system software, and users on this forum (Adrynalyne, Blazingwolf and others) have essentially cracked open the leaked ROM, (which unlocks your Omnia's GPS capability) and improved the speed and capability of the phone greatly. It may seem dangerous, but as long as you follow the steps required and have a stable PC, it will go fine and you will be happy you did. I didn't want to at first, but after I took the plunge I haven't looked back. It is by far the best upgrade you can do to your phone - and it's not like you're paying for it! You can look in the Omnia upgrades section of this forum
here and theidoctor.org
here for more information.
After you get the custom software, you'll want to install programs, games and utilities. I highly recommend
Microsoft My Phone, which is a free service from Microsoft that allows you to backup and restore your contacts, text messages, tasks and a few others after a hard reset or ROM flash. It can be downloaded from that site; it is also preinstalled in some custom ROMs. The main other apps I use are Facebook (created by Microsoft), Pandora radio (google it if you don't know what it is, it's great), YouTube & Google Maps (official from Google) and Kinoma FreePlay (free version of Kinoma media player, I think it's better than Touch Player for listening to music although it is a bit gimped since it's free). Games are a whole other story, I could write another paragraph about those, but the main ones I use: Kevtris 2 (free tetris clone), Asphalt4 (great 3D racing game), G-Invaders (one of many that uses the rotation sensor in your Omnia, very fun), DopeWars (text based drug dealing game
) and Omniano (virtual piano, also very cool).
Many other applications exist to increase Windows Mobile's finger friendliness and add convenience. Samsung's Favorite Settings application makes it easy to change settings and turn on/off some features. I have it mapped to the Main Menu button so it's always easily accessible. You can look around for replacements to most built in apps, like iContact for contacts.
Windows Media Player is actually able to play many of my video files but I hate it. I'm still looking for a decent replacement. You could rip your DVD collection on your computer then copy it to your Omnia, but that's a completely different guide than this one.
Before I end with software stuff, you'll need to know how to install software if you don't already. WinMo software comes packaged usually as a CAB file that you save somewhere on your device and then open, either selecting it from Opera's download list or by browsing to it with the File Explorer. A little hint: when asked where you want to install, it's best to install to My Storage (8GB external memory) unless the application's FAQ tells you to install to My Device (the "onboard" storage - where the system software is installed). You want to avoid adding to My Device because if it fills up, your phone won't work properly. Software also can be delivered as an EXE file that is designed to run from your computer with and install to your phone while it's plugged into the computer.
Getting outside of the software, you'll want to pick up some sort of case and screen protector for your Omnia ASAP to avoid getting any scratches. Mine's picked up a couple while I'm waiting for mine to come in the mail which upsets me a little bit.
That's all I can think of for now (I will probably add more later but I'm pretty tired, I should be sleeping haha) but as I first said, there's so much you can do. Search around for other modifications and programs, and just try them out.
Omnia Hub is one site where I get some of my software from, and since it's specifically for the Omnia you won't have to worry about compatibility.
random tip -- the Omnia's video editor program will not accept videos above 640 by 480 resolution. If you go into Omnia's video recording mode, you will see on the left side of the screen a button with 6 numbers on it, which is the resolution. It records in either 640x480, 320x240 or 176x144. 320x240 is the best, since it records at a 'natural' frame rate (30 fps) and provides the best quality. And of course, the video editor can't work with the 640x480 videos.
I hope this helps, and please feel free to ask as many questions as you want (after using the forum search and Google, of course). The community here is very friendly and you'll likely get the answer you need. Enjoy your phone!
another thing I just thought of (like I haven't said that before) - With all of the customizing you're doing (you should be) you're bound to run into some problems here and there. Don't give up though, you're likely to fix it or find a different way to do whatever isn't working and you'll be happy with what you can do. If you want a phone that "just works" 100% of the time and you don't want to have to solve problems, save yourself the trouble and get a dumbphone. That's not meant to be a turnoff, but just keep in mind that you have to put in a little effort though it will pay off in the end.