My last trip lasted 30 days with Nikki and two other friends. During that trip we made new friends, did lot of sight seeing and had fun and it was overall a new experience for me. On a solo trip I wouldn't have gone to Borobudur, Prambanan, Mt. Rinjani waterfalls or Gili Trawangan.
Nikki feels annual trip to Indonesia is too much for her (we will be on a 2-1/2 months trip to India later this month). So my next Indonesian trip is likely to be a solo one. One of the advantages of a solo trip is you can make decisions quickly and can have maximum flexibility. The downside is, especially for introverts, awkward social moments.