Deck boxes are great for starting out or re-entering the game. They are also good for getting cards that you need in numbers (Holon's Electrode from EX: DS theme decks). Even EX: FrLg Pidgeot, one of the most broken cards in the format, can be obtained from an EX: FrLg theme deck.
Some sets used to have enough theme decks to get necessary cards from. Base Set, Base Set II, and the Gym sets had 4 theme decks.
Starter kits are cool because they often have unique cards. The EX: Starter kit contained Latias and Latios. They also reprinted many cards, ranging from Ruby/Sapphire to Dragons. All of the reprinted cards are currently allowed in Modified.
I generally buy boosters in boxes. It costs $72 for 36 booster packs. It comes out to $2 per booster pack, saving $46.44 in the process (normal price is $3.29 per pack), without including sales tax. It also guarantees that I get several Pokémon-ex, as well as a high chance of getting a Shining Pokémon. Lately, I bought a box of EX: LM. I got Mew-ex, Banette-ex, and Shining Regice.
If you can't afford a booster box, I recommend that you split a box with some friends and battle to determine who gets the box topper.