This blog by Amy Savage is a good starting point for any lawyer thinking of leaving private practice to go in-house.

I'd add a number of other points:

Don't forget that you'll become part of an overhead. The legal function in-house is fundamentally different from being a fee earner. The business doesn't revolve around you unlike when you're a star billing BigLawyer. Controlling costs is a major consideration in-house. There's pressure to do more for less always.

You're exposed to everyone in the business who needs legal input. You'll be used to dealing with the more senior and sophisticated people within your clients when in private practice. Now you'll find you need patience sometimes to explain quite basic legal issues to less experienced non-lawyer colleagues. Occasionally you'll need to be quite assertive to ensure you get listened to and your advice followed. Rarely you could find yourself with the need to escalate / report colleagues who appear to be about to ignore your advice and take unacceptable legal risks.

Given these negatives, what's so great about being in-house? As Amy indicates, it's all about becoming close to your client, understanding its business in depth, and being able to add real value.