To answer the basic question in the first post: Yes, I think it's a rather excellent prestige class: the healing eldritch blast ability shouldn't be overlooked in terms of usefulness to the party, and if you choose the right cleric spells each day, you have a nice range of buffs and debuffs to rely on to assist.
Cloistered cleric / warlock is a pretty good combination, but only if you're willing to give up the hit die (d6 vs. d8) and gain an even worse BAB. Also, the other reason to stay a true cleric would be if you plan to ever utilize heavy armors, although this will disrupt your incantations via ASF. Also, keep in mind your Lore modifier will be rather low, as the levels you increase in ED won't advance it. Still, roll a 20 on the check and you may gain some useful info for free ...
But if you plan to walk around in light armor or less anyway, or use only simple weapons ... then you may not care about what you're giving up as a cleric in order to gain Lore and the Knowledge domain for free.