I am not a practiced reviewer, but I did install it and here are some impressions.
If you are having troubles with Civ5 turn times AND you have a bunch of background processes running all the time, this program could help you.
If you just plain begrudge every cpu cycle devoted to anything that is not your game in progress, this program understands you and agrees enthusiastically.
If you are nervous about identifying what processes are vital or not, it tries to help you along with that a lot, but you are ultimately on your own. It does want to help you put things back if you break them, and that is very good.
It knows Steam games, and that is good for our usage. The first thing it did that I noticed was turn off the WinAmp in the background. The next thing that happened was I found the list and put WinAmp back on. This is contrary to the obsessive begrudgement mentioned above, but it shows that the program knows we will have such priorities.
It is klunky coming back from gaming mode. Some processes just don't like being shut down hard and then turned back on. FireFox and my screen saver randomizer are two of them. Coming back from gaming mode leaves semi-abandoned dialogue boxes for both on my desktop, but they easily dismiss.
There is some improvement. I don't have the skills to characterize it numerically. It claims 40% on my system, but that is not at all true and it doesn't explain that number either. It is noticeably sending updates to the graphic card faster. (part of where my performance seems choked) An old save file of a game late in progress still had long turn times, and I didn't remember that game interactively enough to get a good subjective impression of how much improvement there might be. Subjectively it seemed better. I started a new game and the ancient era and classical era turns do have a certain snappiness to them which is noticeably better.
It didn't break anything for me. It has decent reviews out there and seems safe enough. If you fall into either of the qualifying categories above you might just find it of use to you. If your real problem is something other than background processes, it may only give marginal relief, but if your desperate, any little bit helps.