Not necessarily. However as long as you get through the gate before boarding starts you will be allowed on in the first group. If you are bussed to the plane however, that only means that you get on the bus first. Don't waste your money unless for some strange reason it is very important to you to get on the plane before everone else.
BenDToy is spot on and easyJet do say exactly that on their website when purchasing Speedy Boarding. You need to get at the gate when boarding starts and you are in the first group called for boarding. Obviously, if you are bundled onto a bus first, you are guaranteed to get on the bus first, but not off of it, so a wider choice of seats is not guaranteed.
If you read the easyjet reviews on this site http://www.airlinequality.com/Forum/easy.htm
whilst some people 'praise' speedy boarding, a lot can't see the point of it. Personally I've never used easyjet, so can't venture an opinion.
Personal experience as follows.
It is not the 'gate' one has to get through, it is security. Provided one gets through this to the departure lounge by about 20 minutes before the time of departure, one gets called first if one has bought Speedy Boarding.
The rest of the folks in "cattle minus class" get bundled into 2 blocks - A Boarding and B Boarding. Getting 'A Boarding' depends on getting to the initial Check-ins first.
In reality,chaos ensures when the first announcement about boarding is made and everyone stands around the gate, making it very difficult for the Speedy Boarders to get through. So sometimes they just load the plane.
In summary a ridiculous system - the sooner they drop it the better. The best way to get them to drop it is to refuse to subscribe to it. Power to the People (customers).
In addition to above answers, may also depend if you feel you would gain an advantage when the flight may not be busy. Went Easy Jet to Lisbon in October, and we were very segregated from speedy Boarders, but in the event the flight was quiet enough that I had a spare seat to the side of me each way anyway.
They should call people with young children and difficulties boarding first. Hover near the desk near the time and have a word with the people boarding so they can help you get through easier.
Further to other answers, "boarding" is in the following priority order;
1) Speedy boarding and passengers with young children or requiring assistance
2) Group A - people who checked in on line
3) Group B - everyone else
Note - you can only check in on line if you have no hold luggage
If you check in online make sure you have got a bar code on the confirmation - mine did not have and security sent me back to Ryanair check in to get a duplicate issued.
Further to buildersmate's answer - I do mean the gate. At many airports EasyJet process passengers through the "gate" and then collect them in boarding groups to await boarding. This happens at for example Liverpool and Amsterdam and many others. In my experience they normally stick quite rigidly to the boarding rules, unlike many other airlnes. People who try to get through ahead of their group get bumped back without ceremony, and I personally like this system. Probably because I usually check in on line.