OK, final verdict - the Mugen 1800 mAh battery is much better than the 1300 mAh stock battery and well worth the investment. Once properly conditioned, 24 hours with moderate to heavy use was easily done (GPS on, 2 email accounts on hourly check, an hour or so of newsgroup reading, 1/2 an hour of gaming, an hour or so of web browsing, 1/2 an hour or so on the marketplace, a fair bit of texting and a 20-min phone call in a bad signal area). With fairly light use (email accounts on manual, a few texts, just the occasional 5-min check of Twitter/Facebook and brief web browsing), I've managed over 40 hours and I suspect 2 days is well within possible - I tend to leave GPS on out of habit and have brightness on auto. Knock the GPS off and have brightness on low and that'd save a bit more again.
Obviously having a smartphone you don't use a lot of the functions of isn't ideal, but if you go away for a long weekend and forget your charger, it's good to know you can make it through (if you have the right battery!).