Depending on volume, I think the time to start at stars is when you hit (NL25 with larger volume) NL50 - You then get to earn a little something decent back via the VIP system, to make up for the harder field.
I agree Pokerstars (now that FT is not running) is by far the best software, but there are some up and coming rooms that are okay and bearable.
The best rooms to choose are any that also have casinos (888, Ladbrookes, iPoker etc) as part of the software, as this attracts gamblers rather than just Poker players.
In my personal opinion, iPoker network is awful the software is slow and unresponsive. When choosing tables the lobby takes so long to refresh at times that when you open a table the players are are about 50% different. Depending on the skin some of the support is almost non-existent also, and many are not poker savvy at all. If you do choose iPoker then pick one that has grounding in a real Land based company like William Hill, as these are usually the ones that have best support. And also you can just go down the local bookies (if you like) and deposit/withdraw.
Boss Media (Poker Heaven etc), and Entraction (24hr Poker etc) have nice looking software, but as for what it's like to grind I can't comment as I am yet to grind on any of these networks, but ask around the forum and I'm sure others will be able to help you with these.
888 Where I am currently grinding has decent software (very similar to Stars), the VIP system is a complete joke, but when you consider you have the FDB and then you can get a reload bonus EVERY month by just simply withdrawing and depositing again it kind of makes up for this as you don't expect much in the way of VIP benefits at the micros anyway. The animations and sounds are no where near the quality of Stars (obviously) but they are still liveable - I am yet to experience any connection problems either (touch wood).
I've never liked Party Poker, they have out-of-date polices, fees for everything, and when I did play there for a while, I had connection issues and very unhelpful support.
Cake network again is pretty lame, old out-of-date graphics, and just sitting at cluttered tables makes you want to tilt. Plus, last time I played there (around a year ago) the traffic was extremely low outside the normal peak poker hours.
There are others obviously but I think these are main ones and this post is already probably to long. And I haven't really played at any of the others recently enough to comment anyway.
Hope that helps a bit