Think of beguilers like this: Almost by definition, you want to be casting spells more than you want to be attacking-to-generate-focus-to-cast-spells. Because if you'd be better off just attacking, you could do that at any time!
The beguiler mechanic works like this: If you are casting a Deception spell, you pay the focus cost normally. But for every target you hit that has an affliction (including grazes, and INCLUDING the affliction you might have just applied with this very spell cast), you get 5+PL points of focus back. If you hit multiple targets with a relatively low-focus-cost spell, you GAIN focus. Even for higher-cost spells, if they hit multiple targets, their focus cost is significantly reduced. The two stars for this: 1) Eyestrike (only costs 10 focus, at max level you gain 2 focus (5+PL 7) only hitting only 1 target, and if you hit 2 or more you rapidly turn a profit). 2) Phantom Foes (costs 20 focus, but affects a MUCH larger area, so it is very easy to rack up a lot of focus if the board is full). Play your cards right, and you should be able to spend most of the fight casting Deception spells without needing to stop and make weapon attacks.
This shines in particular in multiclass with casters. Normally that would be a bad choice because spellcasting from the other class doesn't generate you any focus. But if your primary goal is casting Deception spells mostly/exclusively, you don't NEED to generate much if any focus to do that, so you're more or less free to cast either kind of spell without worry. Psion has a very similar friendly-to-caster synergy because they generate focus even while spellcasting, but most people still prefer Beguiler overall for that role.
This also can be friendly for tanks, because there are a ton of fast-casting Deception spells which even a tank can safely cast without worry of interrupts, it plays well into the tank's role of board control, and the tank's typical downside of slow action speed and long recovery is mitigated by the generally long-term payoff of long-duration debuffs, which don't need huge throughput to get coverage. On that note, Borrowed Instinct IS a Deception spell, and it is amazeballs-effective for a tank.
So what's the downside? The chief downside is that beguilers' focus generation from attacking is poor compared to vanilla ciphers or ascendants. With Draining Whip (the default, and the way you should basically always be thinking about your focus generation), vanilla ciphers get 100% Focus per point of damage, Ascendants get 125% Focus per point of damage, and Beguilers get 75% Focus per point of damage if the target has an affliction, or only 50% if the target does not have an affliction (although if you're in this situation the fight should be well advanced enough that you should at least have Flanked, so it's your own damn fault if the target isn't afflicted). So if you wind up using up your focus, and either don't have enough to cast the Deception spell you want, or if it's late enough in the fight that there's no sufficient cluster of enemies to generate positive returns via your Deception focus mechanic, then you will be regaining focus more slowly than you otherwise would. This is partially mitigated if you're saving up to cast Deception spells, because you will need to still save up the full price, but you'll still be getting some of it back even if you only hit one target.