GW's loyalty program was before my time, but I've tucked away enough Mantic points for at least one of their lower or mid tier rewards. Would have to plan out future purchases to see if it's worth cashing in early or saving further. So no, I haven't actually
used one, yet, but I'm a fan of such programs, generally.
"Spend more, win more" tends to alienate FAR more people than it encourages, so powerful exclusive units are out. Even if balanced, I (and I imagine many others) don't like the idea of exclusive models with exclusive rules. Alternate sculpts of existing models, though, and non-model or even non-gaming items (anything from custom dice and objective markers to game/faction pins or patches to coffee mugs and bottle openers) would be of interest.
As for rotation/phase-outs, I would imagine that models also going up for general sale would be best. A lot gets invested in a sculpt and its subsequent casting, which you don't want going to waste. You'd want to cash in on any overstock or potentially missed sales, if it proved popular but difficult to obtain, initially. New or occasional customers with a shallow pool of points might feel cheated, though, if the items just disappear after a set time. The same is true, if to a lesser degree, of non-gaming items with outsourced production. Don't know how long I'd make the period if you were going for at least temporary exclusivity, but I wouldn't suggest making the rewards limited runs - at least not
exclusively.
To work a completely different but potentially relevant angle: If items are available for purchase via money OR points, know that you're putting an exact (if variable) dollar amount on customers' loyalty. It may not happen, but "I bought an army and they gave me a cool character sculpt and a key fob. How nice!"
could turn into "I dropped $500 and they said 'spend another $100 and we'll toss you a $3 mug.'
Gee, thanks..." Something is generally better than nothing, but token gestures can backfire - got a watch for lengthy service to your company? How nice. As severance pay? The bastards! Happy customers are gracious customers, though, so a well-meaning startup is less likely to run afoul of their customers with such a program than an ambivalence-breeding behemoth. Likewise, the greater the perceived value of normal purchases, the more loyalty rewards seem like icing on the cake.
Look at Mantic's program - based on the Mhorgoth model, which can be purchased with money or points, one Mantic point is equal to about $.50. He costs 25 points, so it's very much a case of "buy
several armies, get a free
HQ for one of them... with your next purchase." I have
piles of Mantic boxes and I still couldn't get him for points. Since "big armies, low cost" is one of their major selling points, though, I'm inclined to simply continue purchasing as I normally would, snagging freebies if and when able, than think "What's the point? I'll just cut my losses and get a signed poster. Wait, I'd have to pay shipping or tack it onto
another order?

that!"