In my opinion, it's better to buy theme decks when you're just starting out. You get a LOT more cards for your money/tokens, even after taking out the energy (which you don't need online because you get an unlimited supply), you'll get lots of cards that go together (namely, evolutions. There's nothing more frustrating than getting an awesome stage 2 from a booster, and not being able to use it because you don't have a basic/stage 1 to go with it) and you also get a free 10-card booster for winning 12 matches with it in the Trainer Challenge, and it's a lot easier to build a deck from a theme deck than to build from scratch. Knowing what you're getting can be an advantage too - if there's a specific card you want and it's available in a theme deck, you're better off buying that deck than buying booster packs and hoping you get it. You could easily spend more on boosters than you would on a single theme deck and not get the card you're looking for.
And don't discount the cards that are in theme decks just because they're in a theme deck - Theme decks all have at least one or two really good, useful cards. The main deck I use right now - which has beaten the pants off a lot of more powerful decks as long as I don't make stupid mistakes (my strength is definitely more creating strategic decks than actually playing strategically) - is a modified Solar Strike made up almost entirely of cards I pulled from other theme decks, including the basic colour decks you get just for signing up, and Rallying Cry, which you can get for free (as many times as you want) with the promo code plasmablast.
At the very least I'd say, any time you want to build a new deck, look at the existing theme decks and pick one or two that use the type (and if possible, the strategy) that you want to use. If you don't know what type of deck(s) you want to build yet, get a variety of theme decks with different types and strategies and see what works for you.
