My family just got back on Sunday from a trip where we used Happy Limo for the first time. We had a pretty good experience, overall.
Positives:
1) It is cheaper than renting a car for the week. At least it is if you are staying on property and don't plan to eat off property. If you want to save $$$ on food, you might want to pack a bag of food from home, and use the (brief) 15 minute grocery shopping stop for perishibles and bottled water. Be advised...fifteen minutes really isn't that much time when you are grocery shopping.
2) Our driver was as nice as could be. You couldn't ask for a better person to drive you around.
3) The car was clean and smoke free.
4) The driver was punctual.
5) You don't have to deal with car rental agencies and their hidden cost scams.
Negatives:
1) Don't count on getting the low price that Mousesavers initially qoutes. They mention in passing the manditory gratuity ($20 rt), but neglect to mention the airport surcharge ($4), early/late hour pick-up fee ($10 each way), and magic kingdom gate fee ($9).
2) While the driver may meet you at baggage claim, don't expect them to be waiting there when you arrive (as implied at mousesavers). You have to call Happy Limo, and make arrangements to meet after your plane lands.
3) If your plans change at the last minute, you get charged anyway.
4) If your flight is late, forget about the 15 minute grocery stop. The 15 minute clock starts ticking when your flight is supposed to land, not when it actually does.
5) It is not as good an option (compared with a rental car) if you want to see anything other than Disney on your trip.
I don't mean to get bogged down with the negatives, but in fairness they deserve mentioning. Bottom line...we had a good experience, and would use them again on another trip.