I used to workout this way. It's probably not a bad idea until you build some good experience. The problem I ran into is that I woul dget obsessed about how well my workout went compared to my last one.
Anymore I specifically go to the gym and intentionaly do a different routine every day (i.e. Day 1 is chest and tri's. This week I may start with flat bench, then do decline dumbells, then incline dumbells, next week I might start with incline bench, then do decline bench, then do flys on a machine, etc.) What's most important is that you are working the muscles with proper form.
I pretty much pick an exercise and then slap however much weight on feels like I can do between 8 and 15 reps. My goal is to do, at a very minimum, 6. I can usually tell on the first rep if I cab get there. If not, I stop and take some weight off. By the end of the set if I'm under 8, I lower the weight. If I'm over 15, I increase the weight.
Someone feel free to correct me if they think I'm wrong, but if I had to summarize some principles, I'd say:
1) Eat right 2) Dial in the right over all load per muscle group (meaning at the end of your session, you will have worked out enough, but not too much. 6 sets? 15 sets?) 3) Keep most working sets in the 8-15 rep range. 4) Do what seems to work and keeps you motivated and injury free
If doing the same workout every week following a steady progression where you get to see the weight move up periodically accomplishes #4, then go for it! 4) Work all of your muscles regularly.
Yes and no. In a way, all lifting is subjective. You can do, for example, 4x8 bench press every week at 190lbs, and progress when you hit 8 reps on that last set, which is certainly an objective way of deciding when you are ready to move on, but deciding that 4 sets is the right amount instead of 3 or 6 or 12 is fairly subjective itself.
Different routines are going to workout for different people, and there will always be a degree of objectivity vs subjectivity in the workout. I wouldn't advocate my approach to anyone that it doesn't work for. I pretty much enter a workout with a goal in mind of how many sets I want to hit per body part. As I approach the end of the workout I listen to my body and determine if I've done enough or if I need more.
How do I know if I'm progressing? I measure my body parts, weigh myself on a scale, and look in the mirror. Since this is the body building forum, that would probably be the most important part. Beyond that, I do a test workout every 3-6 weeks. For example, I'll hit the flat bench for 3x8 and the beginning of the workout. Hopefully I can lift more than the last time I did this test.
Again, this is what I've adopted after years of working out. I find lifting for the burn has yielded more results (and fewer injuries) than chasing a log book. FWIW, as a running coach, I only use half this approach. I give the runners a workout and a time goal, but I prefer them to either leave the watch at home, or run a route that they don't know the distance of. Chasing numbers, I believe, leads to a lot of over-training.....and to follow up on Lonnie's post, I don't mean over-training in terms of volume per week, but rather in terms of lifting loads that can injure you, or with bad form.