Slamet & Sumbing

Not me for sure, though I did climb about halfway up Slamet from Baturaden.
I did that to look for the endemic leaf-monkeys, so there was no point in going all the way to the bare top.
Also, what I took was a very little-used trail, and I met no one else taking it.

The main routes used by local climbers (3 routes to Slamet, 2 to Sumbing) are well described in a newish (2003) climbing book "Mari Mendaki Gunung: dari Leuser Sampai Carstenz"
Get it - there's little to add to the info in there.

was there 10 yrs ago. slamet is the difficult with unclear path and high steps. Try to attach with local trekker. The last one hour to the top is unstable lava rocks, no more vegetation. It's better you camp on last base and reach the summit for sunrise
Sumbing easier compare to slamet but beware of the sandy slope as the wind was strong and difficult when you going down. If you reach big square stone, then you already close to summit
Both mountains have crater

i've been to mount slamet 3 times and sumbing just once
slamet - all from Bambangan village. It is easy to reach Bambangan. From bus station Purwokerto, take a mini bus to Bobotsari (around 1 hour, cost around rp5000). Get off at Serayu junction (just ask the driver or 'kenek' to stop at Serayu, it is on the left side of the road)
from Serayu, take a car to Bambangan village (1 hour, around rp10.000-15.000).
this car will stop in front of "pos pendakian slamet" (registration post). Here you sign the registration book and pay the ticket (for you maybe rp5.000). Or you can also hire guide here (dont know the fee).
If you just do it yourself, just follow the asphalt road, up to the gate with the sign "gunung slamet". From here, follow the track, turn right (so the dry river will be on your left) and continue up through cabbage fields, the forest.
it took me around 6-7 hours walk from the village to reach the camp of Post IV (it has a small hut). Here i make camp. From here, it still takes 2-3 hours to reach the top.
Don't climb slamet in rainy season, you will see nothing except heavy mist at the top. February usually this mountain was closed

Sumbing, hikers usually choose to climb from Garung village as its trail will straight up to the top
but i choose from Cepit village. Your first destination is Parakan (take a bus from Semarang to Parakan). From Parakan, hire an ojek (motorcycle-taxi) to take you to Cepit trail (around 30 minutes, maybe more than Rp50.000). Start from the end of asphalt road, just follow the dirt trail until you reach the area of Watukasur (around 5-6 hours of walk). Make a camp here, and go to the crater in the morning. Cepit trail will end directly at the crater, not the top.

Thanks for that, especially the heads-up on the book. Can it find it Gramedia?

Once I finish here in Timor in a couple of months, I hope to take some time off and ride a motorbike from Bali to Medan. I'll be asking you for more info on volcanos and national parks...

I bought my copy of that book in Gramedia - in Jayapura.
You should be able to get it in other Gramedias, though of course in Java, where climbing is most popular, it may have sold out more quickly. Just checked their website, no luck there, unfortunately!
For general national park info, consider oredering either this book or this one online while still in Timor - believe me, they are worth it if you are into visiting parks!