You should definitely go. But I want to add another point not being mentioned: Make sure you have the offer officially in writing before telling your employer.

You should definitely go. But I want to add another point not being mentioned: Make sure you have the offer officially in writing before telling your employer. I know of a number of people who were burned when something fell through on the other end and they were left in the lurch. Also, business is business... Any software consultancy SHOULD understand the inherent risks of a client poaching and/or bringing work in-house. If your current employer will "be really mad" for you advancing your career you should get out ASAP as they don't care about their devs.