I've been looking into this and have some concerns about its usefulness. Accoring to
this article at GPSReview.net, both Garmin and TomTom get their data from the same place - OPIS. When I last used MSN Direct on a Garmin, a lot of the data was 1 or two days old, and some was 5 or more days old.
You can also get MSN gas prices
here. As you'll see, the list of prices includes 30 gas stations, sorted by price in ascending order. At the time I'm writing this, the data for the first 7 entries (the 7 lowest prices) is 1 or more days old. The prices at any of these stations could now be easily higher than the #8 station that has price data from today.
According to the above-referenced article:
TomTom says that “60% of Stations will report Regular Fuel price updates every 48 hours”
If the data was a snapshot - meaning that all of the data was two days old - this wouldn't really be much of a problem, as the station with the lowest price two days ago probably has the lowest price (or close to it) today, regardless of what has happened since then. But it's not a snapshot. Some data is from today, some from yesterday, and some even older. With the way prices are changing these days, a mix of data from today, yesterday, two days ago, etc. is virtually useless.
What would be much more helpful is some sort of approximated snapshot - show me the data for the stations as of yesterday, so I can compare apples to apples, instead of todays oranges to yesterdays apples. If some stations reported today, but not yesterday, estimate their "yesterday" price based on historical data for that station.
I still may try it myself, but it's not so much the $15 subscription I'm concerned about, but the added cost of being directed to a higher priced station due to stale data.
I looked into the possibility of scouring the MSN site or the GasBuddy site to develop a POI list for my own use, but the prices that appear on the web sites are all displayed as images (not text) - apparently to prevent people from doing just that. While it may be possible to decypher what they are doing, and obtain the pricing, I'd much rather spend $15 and let somebody else to it.
If I do decide to try it I'll certainly post back here with some sort of review, at least as it applies to my area.