First - have the ET tips gone downhill in quality?
Second - how do the Hakko fx888d tips compare? Are they better? worse? last longer or shorter?
Regarding the ET series, quality has gone downhill (COO = Mexico, which has a higher defect rate than tips they make in other locations; read on).
There are alternatives though, so you're not 100% stuck with Weller if you don't want to play Tip Roulette (
Plato/Techspray offers an equivalent for the ET series for example;
here).
As per Hakko, they're definitely better IME (thicker iron plating = last longer), so I tend to recommend the FX-888D over the WES51 if the buyer can live without a knob.
One of my clients uses Weller WD series irons, and he has complained about the $$ he spends on tips. I won't go so far as to say I hate using the WD when I'm on site, but I am not impressed with the wettability of the tips.
FWIW, the LT series tips made in Bosnia are a crap shoot regarding quality, and unfortunately, they're the more common profiles, such as the most commonly used chisel sizes (~30% defect rate IME). Yet the tips made in Germany, Japan, or the US are fine as a general rule.
Plato/Techspray makes
LT series compatible tips however, so there's a work-around for these as well.
They work quite well IME, and the performance has been consistent so far. Even found equivalents made in China on eBay.
... [snip]... clean, then use tip reactivator...[snip]...
Then tin them with fresh solder regularly enough to avoid them becoming crusty again.
+1 on this.
As mentioned, this is only meant for occasional use in order to rejuvenate/extend tip life (rinse with plenty of fresh solder before using it again).