I have to admit both prejudice and ignorance about rented lists. I've never used them, due to a strongly held belief that has served my clients well: you should not send e-mail to anyone who has not actively requested e-mail from your company. Because e-mail is personal and intrusive, people react strongly to unsolicited e-mail--far stronger than they do to postal mail, slightly less than they do to telemarketing calls. Even if the short-term ROI works out, is it worth it to have some portion of your potential customers (and their ISPs and the blacklist managers) consider you a spammer?
I've had plenty of experiences with people who claimed they never signed up for e-mail, despite a data trail indicating they did. So I can't imagine people who opted in for "e-mail offers from our partners" or "e-mail offers related to subject X" welcoming all e-mail solicitations that result.
Inducing e-mail opt-ins is also a slippery slope. If you provide incentives for opting in, such as sweepstakes entries, content access, prizes, etc., you end up with subscribers who may resent you and your presence in their inbox.
Even updating e-mail addresses through append and change services is dangerous. If consumers change their e-mail address, do they still want to receive e-mail from you, or did they change e-mails to stem the flow of commercial e-mail? As Miller Brewing recently learned, a consumer may opt-in but provide a fake e-mail address. Correcting that error through E-mail Change of Address (ECOA) lead to not only a very unhappy customer but a barrage of negative press. Consider instead the approaches suggested in "Battling List Attrition."
I believe that in order to build your brand, you need to build your list with care. And that means focusing on quality opt-ins--those who actively request your e-mail--rather than the size of your e-mail list.