Even professional magicians use Svengali decks. With correct handling and some imagination, you don't even need to arouse suspicion in the audience that it's a gimmicked deck.
Get one in the 'flavour' of your standard decks (typically Bicycle), and use it wisely.
The standard use is as a forcing deck, but there are also some more imaginative uses of the deck such as total colour changes—changing the deck from a red back deck instantly to a blue backed deck, and then back again—(get a red and a blue Sven deck, and you will pretty much get the idea). There have also been uses of the Sven principle for many visual effect decks, which I obviously can't name, but it really is the PRINCIPLE of the gimmicked cards which is important, not just the 'one-trick-pony' which most people associate with this deck.
I can't say I agree with the 'Gimmicked deck = David Blaine". David is actually a very proficient and skilled card magician, and most of his work uses regular decks of cards and deft sleights.
So, in short:
1. The Svengali deck is a very reliable and simple way to force a card
2. It is also an excellent all-in-one routineable trick in it's own right, many books are available on the subject
3. It appears as a totally ungimmicked and normal deck. The spectator CAN handle it—with supervision, and they can even cut the deck to shuffle. In fact, there are at least 2 routines I use where I never touch the deck at all—the magic is done by the spectator.
4. The Sven deck is versatile, and should not be considered a gimmick, more of a utility. The Svengali principle can also (and IS also) used widely by professional mentalists
5. Misconceived as a gimmicked 'beginners' magic item, the Sven deck is sold under the Marvin's brand in most good highstreet toystores for a £5er. The revered Andy Nyman (to name but one) used to demonstrate Svengali decks for Marvin's at Hamley's Toystore in London. And look where he is now
6. The deck is wholly self-contained. Once the principle is revealed by purchasing the deck, it is not impossible for a creative magician to forge his own custom Svengali deck from regular cards
7. The cost will vary, depending normally on the actual type and quality of the cards used. A deck of Bicycle Svengali cards will typically cost £6
from Cards4Magic.
Not much more to say really, except Svengali decks are an excellent tool for the beginner, and an indispensable utility for the professional.
As with all magic—it's all in your presentation. Gimmicked decks are generally self-working, but it's the Svengali deck's massive expandability and 'thinking outside the box' which have kept it at the forefront of card magic.