I've been teaching a variety of workshops recently, some of which are Scrum. One of the questions people have is: Can we do this partway?
No, not Scrum or any other agile lifecycle. You either do it all or you're not doing agile.
You can work in timeboxed iterations. But if you haven't gotten to done at the end of the timebox, it's not agile. If you don't do retrospectives, you're not doing agile (I don't see how you can improve for the next iteration if you don't look back at this most recent iteration). If you have to have an entire iteration (or two or three) of just architecture, with no working product, it's not agile.
Timeboxes are helpful to people. But just timeboxes don't make a lifecycle Scrum (or any other agile lifecycle). One group asked me “What if we keep the requirements and architecture phases? Then move into timeboxes where we get to done? Is that Scrum?” No, it's quite a useful incremental lifecycle. But it's not agile.
I understand why people would prefer to dip their toes instead of plunge in. If you've got an organization set up to do phased development and everyone is judged on their ability to meet their phase deliverables, it's pretty darn scary. Especially if you don't have automated tests and you have a legacy product. How can you tell you're done at the end of a timebox?
The problem is that agile won't solve these problems unless you put them on the product backlog or remove them as obstacles. Agile makes them visible.
For a project team, plunge in. Or, decide not yet. But don't dip your toe and not commit to really being agile. You're setting yourself up for complaining later.
(Added later because Joe was confused) Plunge in all the way. Go to agile. Commit to it. Or, commit to an incremental lifecycle. Commit to something. And, call it what it is. Don't call it agile if it's not, and then complain later that it doesn't work. Don't call it incremental if you don't finish features, as in done-done-done. Know what you want to do, commit to it, and then evaluate your progress or success. (Is that better, Joe?)