Two questions you may want to ask yourself before you get Invisalign are:
Do you mind having your teeth filed?
Do you mind wearing a retainer for the rest of your life?
These are two things my dentist neglected to tell me before I went through the whole process, and from what I've read on various forums it looks like this is standard operating procedure amongst most dentists. These would have been deal-breakers for me if I had known as my teeth were only slightly crowded on the bottom and it wasn't that big of a deal to me.
Also, the initial treatment plan may not completely correct your teeth and you may have to go through a "refinement" process, which my dentist said is no extra charge. From the various forums I've read, this also seems to be normal procedure - and most people do end up needing to go through the refinement process.
To answer your specific questions, these are my answers:
a) They are not completely invisible - but it would be a bit absurd to expect them to be. They are clear plastic, so if someone is pretty close to you and they are staring at your teeth they will probably notice something is up. They will either see the aligner (or at least notice that your teeth have a shiny sheen to them) or they will see the "handles" on your teeth if you have to have them put on your front teeth. But most likely in a normal everyday situation a person probably would not be able to notice them on your teeth, but they may notice that you talk a little different if they interact with you on a regular basis, which leads to...
b) You will usually adjust enough to not talk with a lisp after a few days (or a week) but IMO you will always talk a little funny. My coworkers all know I have them and they definitely notice my speech is different while I have them in. Also, in long conversations you get a little exhausted from talking with them in, so I usually take mine out in those instances. I also find I frequently get "tongue-tied" with them in. It can definitely be frustrating at times.
Basically it will come down to whether the inconvenience will be worth it for you. Everything has it's ups and downs and Invisalign is no different - they're not perfect, but once your done hopefully your teeth will be straight. Expecting to go through the whole process without anyone finding out might be a bit of a stretch though. Remember, you have to take them out when you eat, brush your teeth and the aligners after you eat, etc...