[dj_mysha]

Most 6.5 ROMs have TF3D and Titanium included, so you just need to turn one off and the other on to switch flavours.
For me, the biggest upgrade from 6.1 to 6.5 is that flick scrolling has been implemented throughout the entire OS. To give you an idea of what that means, think of the nice scrolling experience you get in Opera. Wouldn't you like to have that in Contacts or in File Explorer? Well in 6.5 you do! And in every other program as well. This really increases 6.5's ease of use factor to something well above that of 6.1 when it comes to navigating the OS.
Titanium is another big upgrade. I've been rocking FuFu now for over 3 weeks and I haven't missed TF3D at all, which is saying a lot! Once you get the weather and favourite people panels in there (search here or at XDA), the user experience improves dramatically. Titanium also gives you much more freedom to choose your desktop wallpaper. I found that with TF3D's text being white with no stroke, I was limited to only dark coloured wallpaper, whereas I have yet to find a wallpaper that has interfered with the legibility of Titanium's text. Give Titanium a chance and you may be surprised at how much you like it. I know that I was!
Otherwise, battery life, speed, radio performance and stability have all been comparable to any 6.1 ROM I've flashed and I haven't run into any applications that I am not able to run. Give the smart folk around here and at XDA a little while longer and I think you'll see much more in the way of customization. There are already a number of themes available, but I think that what's out there now is just the tip of the iceberg.
My opinion of 6.5 is that it is a definite upgrade from 6.1. It may not be the absolute game-changer that we all hoped for, but it does bring a number of significant improvements to the Windows Mobile user experience. I agree with salisburyl in that once your phone is unlocked, it's pretty tough to brick it if you follow the instructions in the sticky, so I'd say go ahead and flash the flavour of your choice and have a little fun with it!
I hope that helps!