Some are great, some aren't. While these textures are high resolution, many of them are not only very different form the originals but also have some serious problems that make them lose their visual appeal.
As of now, the TTEP project is being revised by members of the community, so I would hold off on this version and wait for next release.
The TTEP (Total Texture Enhancement Package) is a massive package containing remakes of the original textures from the Marathon trilogy.
It replaces the original low-quality textures with new high-ress versions with a maximum size of 1024x1024 pixels. I've tried to get them as close to the originals as possible.
New in version 7.0:
Script files have been cleaned up and files have been renamed to make it more clear how to install.
All textures are now optimized .DDS files, with increased performance as well as quality!
Textures can now be scaled down in Aleph One's OpenGL settings. This also makes the TTEP a single size package, there's no longer 4 different sizes.
A lot of the textures have tiny flaws removed, some are completely redone to make them look better.
The Textures are now divided by their games, meaning you're not forced to download and use the Marathon 2 Textures when you only want the Marathon 1 replacements for example.