I never have had a problem with my xp3's. I have 5 of them, they've all been running great for months, the longest one around a year at this point, and apparently a lot of other people have had good experiences with them.
I've heard of problems with every filter. I haven't had luck at all with fluval 04 series, and I know of several people that have had problems with and absolutely hate most fluvals.
I run a bunch of emperor 400's, some of them used, and none of them have failed or had issues, yet I know some people that have bought several and had issues with almost every single one, same goes for penguins.
I hate Aquaclears yet tons of people swear by them, but they haven't suited my needs in the least, I don't even have any running anymore.
I really don't think there's any way to know what filter you prefer without actually trying them out. I've yet to see any company where absolutely no one complained that their filter failed or wasn't right in reviews. There always seems to be a small percentage of manufacturing errors and I guess if you happen to get several models from the same run there's potential of them failing.
As far as the customer service thing, I have no idea because I've never had to call about any of mine. I have seen a few reviews that said they took awhile but they did eventually send replacements.
Either way, I'm personally extremely happy with my xp3s. I think the xp4s are way overpriced whether they work well or not *shrug*.