I think it best to start this review by mentioning what these aerials are not for; specifically, these aerials will not magically boost the range of your network, so if you're having trouble getting a usable wireless connection in a large property or with an awkwardly located router, then I'm afraid these aren't going to help you. Anyone in such a situation may experience a small boost at best with these aerials, and only if both devices (wireless access point and wireless client) are able to fit these external aerials, you are much better off looking into moving your wireless router to a more central location, or investigating a wireless bridge or a repeater (both will likely require your existing router to support WDS!).
What these aerials are good for however is boosting the performance of an existing, stable wireless connection, which is what they've done for me. I was in a situation where my wireless performance fluctuates a bit, largely because of interference from neighbouring houses, so while it gave great performance during the day, this would drop off in the evenings and slow my ability to connect to the internet considerably. After fitting six of these aerials, probably a bit overkill with three on the router and three on the client, I now get better speed at all times of the day. The aerials are 29cm long, but should fit onto most wireless devices with external aerials; remember to spread them so they're not all pointing in the same direction, as this gives the best signal.
So, at £4.15 a pop these are a good way to replace external aerials on devices that may ship with somewhat cheap, dinky aerials. In my case I had a D-Link Xtreme N Gigabit Router DIR-655 - Wireless router - 4-port switch - Gigabit Ethernet - 802.11b/g/n (draft 2.0) - desktop
which, though an excellent router for a small home-network, comes with fairly lacklustre aerials. However, you can get one of these along with three of these TP-Link aerials for less than £100, which will suit most small properties just fine, but if you still don't get the coverage you want then you will definitely need to look at getting two or more devices with WDS support to give you the coverage you need.