What's a prerelease?
Check out Pokemon TCG Prerelease
I attended two Mega Evolution prereleases and it was one of the most exciting prerelease in a long time. The decks were good, they came with really good cards for GLC and played against each other in a very fun way.
In the first, I got the Meganium deck:
Pokémon: 7
3 Chikorita MEG 8
2 Bayleef MEG 9
2 Meganium MEP 1
3 Exeggcute MEG 4
2 Exeggutor MEG 5
2 Celebi MEG 12
1 Dhelmise MEG 18
Trainer: 12
2 Lillie's Determination MEG 119
1 Lacey SCR 139
2 Drayton SSP 174
1 Lana's Aid TWM 155
1 Brock's Scouting JTG 146
1 Forest of Vitality MEG 117
1 Festival Grounds PRE 108
1 Air Balloon BLK 79
1 Ultra Ball SVI 196
2 Bug Catching Set PRE 102
1 Buddy-Buddy Poffin TEF 144
1 Switch SVI 194
Energy: 1
10 Basic {G} Energy MEE 1
Total Cards: 40
and in the second, Alakazam deck
Pokémon: 8
2 Abra MEG 54
3 Kadabra MEG 55
1 Alakazam MEG 56
2 Spoink MEG 62
2 Grumpig MEG 63
1 Jynx MEG 57
2 Xerneas MEG 64
1 Cinderace MEG 28
Trainer: 14
2 Mystery Garden ASC 194
1 Redeemable Ticket JTG 156
2 Deduction Kit SSP 171
1 Repel MEG 126
2 Dusk Ball SSP 175
1 Buddy-Buddy Poffin PRE 101
1 Brock's Scouting JTG 146
1 Kieran PRE 113
1 Lana's Aid TWM 155
1 Ciphermaniac's Codebreaking TEF 145
2 Lillie's Determination MEG 119
1 Emcee's Hype DRI 163
1 Boss's Orders PAL 172
1 Lt. Surge's Bargain MEG 120
Energy: 1
8 Basic {P} Energy MEE 5
Total Cards: 40
I went 3-1 with the Meganium deck, it was really powerful in this format. I beat two Meganium decks and one Alakazam deck and lost barely to another Alakazam in a tight game where I was one turn too slow to get rolling.
With Alakazam, I went 2-1. I beat two Meganium decks and lost to a Garganacl deck.