Regim is easy to use, extremely automatic and does easily some things I haven't found in DSS. For example removing gradient which is quite handy for me since I take photos on my backyard. Every one of them has some gradient from nearby city lights.
On the other hand Regim is quite limited in what it does and it really requires real image processing software to get anything done.
I tried Regim on some of my old imaging data and compared it to what I got with DSS. Here's PanSTARRS comet close to Andromeda processed with DSS.
|PanSTARRS and Andromeda stacked with DeepSkyStacker|
Here's what I got with Regim. After stacking I removed the gradient, adjusted white balance with Regims "manual" tool (I set the b-v value of Andromeda center to 0.63), and did the final adjustments in Darktable. So everything in Linux only. Finally.
|PanSTARRS and Andromeda stacked with Regim|
Now if only Gimp could handle 16bit colours...