Theory is one thing, but if you're going to shell out $25, you'll want to see some real advantage, right? Figure 2 shows you the difference the MINI can make.
Figure 2: Performance comparison
(click on image for a larger view)
For my test I used my trusty ORiNOCO Gold PC card in my Dell Inspiron 4100 running WinXP Home, paired with an ASUS WL-300g Access Point and found a location where I had a marginal signal. The Chariot plot starts out without the MINI attached and you can see where I snapped it on about halfway through the run.
I also ran NetStumbler both with and without the MINI, but didn't see enough difference to make it worth including the plot.
NOTE! The improvement you see depends on many factors and you may not see any improvement at all under strong signal conditions. Better antennas make a difference when the going gets tough, i.e. under weak signal conditions.If your present signal is sufficient, adding the MINI - or any "booster" antenna - won't improve your WLAN performance.
If you have one of the cards that can use it, the MINI will definitely come in handy, especially if you're highly mobile. $25 may seem like a lot, but if you've ever been in your room doing email sitting in the (empty) bathtub because that's the only place you can get a decent signal from the hotel's hotspot, I think you'll agree it's cheap at twice the price!
If you really are on a budget, you can always order the kit for $16, but - detailed assembly instructions nonwithstanding - most folks will be better off letting Aerialix do the cutting, soldering, and shrink tubing.