Blue Shift is at best a short and mediocre follow-up, and at worst is a blandly rushed and unimaginative game.
Within Blue Shift, you take up the role of Barney Calhoun, an ordinary mid-level security officer at the Black Mesa Research Facility, who just so happens to be friends with one Gordon Freeman. In the original Half-Life, all of the security guards scattered throughout the facility were technically known as "Barney", though the genuine Barney Calhoun is seen right at the beginning of that game, hammering his fist against a door. This is one of the first actions the player does in Blue Shift, thus also managing to catch a glimpse of Freeman passing on the Transit tram. During the rest of the game, the player once again witnesses the effects resonance cascade disaster created by Freeman during the original game, and must attempt to escape the facility alive, enlisting the help of various scientists, notably the new character of Dr. Rosenberg. The story itself is perfectly adequately executed, and remains solidly logical, but on its own it is nowhere near enough to support the game's various flaws.
To start with, Blue Shift is almost inordinately short. Depending upon what difficultly to select, it will probably only take a maximum of 4-5 hours to complete the game, and once you've done so, there isn't really anything that would want to make you play through it again. All of the environments and levels are blandly similar in their appearance and design, and are lacking in any of the larger set pieces which were interspersed into Half-Life. All of the enemies you meet are once again all recycled from the original game, such as Vortiguants and the HECU Commandos, and do not encorporate any of the new, "Race X" aliens which made Opposing Force worthwhile. There is also a complete lack of any Zombies, which given the great number of Headcrabs thrown at you during the short campaign, remains somewhat of a mystery. Dealing with all of these is simply a matter of shooting, and because of the semi-auto aim, this is no problem whatsoever.
All weapons are likewise reused again, and further some of the original weapons (such as the rocket launcher and crossbow) are omitted, meaning there is actually a smaller collection of weaponry to choose from. One of the only things which helps to make Blue Shift somewhat worthwhile is the inclusion of the High Definition Pack. This updates all of the character models with a greater polygon count, and also changes their look significantly, most notably on the HECU soldiers, so it is debatable which looks better. All of the weapon models have also been altered, such as changing the MP5 submachine gun to a M4 Carbine rifle. However, as none of this alters gameplay in anyway, the changes are entirely superficial.
Further, as in other Half-Life games, Barney never speaks during the action. Whilst in the original and Opposing Force expansion this fact didn't matter, or was inherently useful to the actual progression of the game, here it doesn't make any sense. For example, there is a conversation with Dr. Rosenberg where he asks if you were sent by his (unfortunately late) colleague, Harold. Since you cannot speak, Rosenberg should not be able to find out if you actually were. Yet his next like of dialogue is "I thought so", as though you had responded. How on Earth was Rosenberg able to deduce that? Sound suffers some general problems as well. Ambient noises seem either too loud, or as if they are being spoken through a scarf, making everything seem slightly muffled.
There are also some irritating graphical hitches here and there for the Steam version of the game, which is now the most likely version you are going to play. In the original game, the HUD was blue, yet here it is orange. When Freeman is seen travelling past in the tram, his character jumps about and occasionally flickers. Similar things happen when Calhoun is inside an elevator with two scientists. Considering this game has now been out for seven years, you would have expected simple problems like this to have been sorted, and although Gearbox Software released an update eliminating these problems for the original version, it has not been instigated within Steam.
In conclusion, Blue Shift on its own is not worth more than a couple of playthroughs at the very most, due to its short and uninventive gameplay. It does seem that Gearbox Software didn't give it anywhere near as much love or care creating it as they did for Opposing Force. Even now on Steam, $9.95 seems rather steep for the game on its own (considering it was $20 upon initial release with Opposing Force included). However, when purchased in conjunction with the original Half-Life or Opposing Force expansion, it exists as an interesting, but noticeably weaker development of the Half-Life story.