That this series never made it past the first episode is one of the greatest losses in the history of gaming.
It is awesome.
I know the Half-Life series gets a lot of credit - as well it should. But the Half-Life series has some serious gameplay flaws - like being forced to complete platforming puzzles without being able to see your own feet, or constant reliance on those annoying and frustrating sandlions, and so on.
This is a Source engine game, and it plays very much like Half-Life 2, just without those glaring flaws that have become even more glaring over time.
This game holds up extremely well. In the days of PhysX, its use of native Source engine physics is still impressive. It's level design is consumate. It's difficulty can be adjusted incredibly well. The voice acting is as decent as you can expect from a game of its day.
Indeed - and in a very real way - if you can look past the unfinished plot, you will see glimpses of gaming brilliance that, sadly, went unnoticed in its day, and largely still does.
If you have $9.99 and love shooters, this game is a must-have, especially now that nearly everyone has the tech in their rigs to max its settings to proper HD, enhance with proprietary GPU driver filters, and really enjoy the experience as the designers intended.
There are some flaws in the game itself, but bear in mind it was supposed to be the 1st of 9 episodes. These flaws are an over-reliance on just one or two types of enemies, for example, and a few bugs in the AI that, understandably, never got patched. Unfortunately, it is just plain tough to tell where the designers were going with enemy design and how these one or two basic enemies were going to relate to what may have come.
All of that said, just drop the tenner and get it. It deserves it, and maybe, just maybe, if enough people show interest, the series will be picked up again.