Lets say if you want to rent a place that's about 512 in size, most landlords are going to charge you about $2 per week. Now, if you are going to spend in SL anyway, and the L300 will be used, that is the same as getting real money back. If you keep within your 512 land tier, there will be no land usage fees, and the average of your premium is about 18 cents per week with the $72 per year. A little better than $2, right?
Also, if you want to share land with another person, if you both have premium and deed your land tiers to a group, then you have 1024 and still pay no land usage fees.
If you plan on using SL a lot and spending regularly, the premium is a pretty good deal.