I didn't find the initial encounter too challenging. There are drow politics to be exploited, many potentially allies to be made, and if all else fails, a distracting demon attack to facilitate escape! I prepared a simple combat card for the capable NPC's (little more than weapon type, chance to hit, and damage), and during the initial section had some of the other prisoners pair off with the PC's and allow them to control the NPC as well. (These relationships became quite strong as the campaign went on, but as of right now only Shushar and Stool are with the party, all the others having decided to leave, or been killed off).
I would strongly advise, if you have the time, that you should design multiple encounters to entertain the party as it travels from place to place. I do not subscribe to the "travel time is dead time" school of thought. Travel time, especially through the outlandish nightmare that is the Underdark, should be fascinating, terrifying, and occasionally hilarious. Drow, duergar, kuo-toa, demons, and all the other denizens of the dark places of the world are at your disposal! Not to mention environmental hazards (I had the PC's traverse a frothing rapid, an ancient crumbling aqueduct, and haunted duergar canals, for example). When the PC's reach Gracklstugh or wherever they're heading in one piece, they should feel rightfully proud of themselves, and you will have enjoyed an awesome opportunity to invoke the environment and potentially introduce other stories to the party.
Although I dearly love OotA, and it has been my main campaign for a year, I have to admit that structurally, the first half is a mess. As others have said, it's essentially a sandbox with the players trying to find a way out before the drow catch up with them. The adventure doesn't ever see to want to admit that, however! You'll want to read everything up to the end of chapter 7 very carefully before embarking on the campaign, and make copious notes as you go on important events, people, and information that should get tot he PC's.
glhf!