The most common type of brachytherapy is the seed/permanent/LDR brachytherapy, where radioactive seeds are inserted which deliver the dose over about 200 days (in the case of Iodine125), after which they have become too weak to have much more effect. The dose is front loaded, and gets weaker as time goes on. This is usually given by itself or with hormone therapy, depending on the risk of the cancer, and depending a bit on the oncologist doing it.
The other type is temporary/HDR brachytherapy, where a single much more powerful seed is inserted for about 10 mins and then taken out, and the treatment is finished when you go home. However, a single HDR brachytherapy session doesn't work well as you need multiple radiotherapy sessions (or a long one as in the case of LDR) to be effective. Originally, 4, or 2 HDR sessions were done, but much more common in the UK today is that one HDR session is given, and the other is swapped for a half dose of external beam radiotherapy (known as HDR Boost). This is usually also given with hormone therapy (mainly for the benefit of the external beam radiotherapy).
The combination of HDR/external beam/hormones is good for high risk patients up to T3b. LDR brachy tends to be used for lower risk patients G3+4, T2N0M0, but can go up to T3a in some cases.
I had HDR Boost, and 2½ years later, I'm at the tail end of recovery and everything is working, except ejaculations are mostly dry (as expected). So far, I'm pleased with the result and in a much better state than I imagined at the outset. It will be another year or so before I know how successful the treatment was, but it's looking good so far. However, I'm aware I was a high risk patient, with perhaps a higher chance of recurrence.