well it depends on your goal, isometric have many useful applications, however like anything in training there use all depends on the specifics. You need to establish what you are trainig for etc etc etc. Isometric training is much more complicated than simply holding a weight or pushing against an immovable force, different methods have different applications. I personally believe they are exceptionally useful and should be used a lot more, fucntional isomtric training can significantly help you improve your lifts. Remember during any lift, no matter what exercise it is, there is always an isometric/quasi-isomtric (quasi-isomtric is a term used to describe a very slow moving portion of the rep/muscular action)portion. By training to increase your strength at these moments/sticking points etc your lift numbers will go up.
To factor them in you do need to remember that they can be fatiguing to the nervous system, and yes in some individuals they have a nasty cardiovascular effect (due to prolonged breathe holding elevating blood pressure). However if isomtirc holding is limited to no more than 10 seconds, and you use a decent work/rest ratio then most of this can be minimised. Remember as well that isometrics will only improve performance close to the specific joint angle,
If you want to use isometrics for developing your max strength (1 rms etc) you may want to try
1) Pushing/pulling etc (gradually) against an imovable object
2)Raising a load and hold at specific angle (sticking points etc)
3) Pushing/pulling a weight from a support/power rack up to a pin/stop, (like a partial)
perform these for no longer than 8-10 seconds per rep, 6 reps per set, with a 5-10 seconds rest between reps. Limit there use to no more than 10-15mins per session
if your after hypertophy then Im afraid that isometric training produces very little growth, however indirectly by increasing your max strength levels you can handle more weight and indirectly help to increase hypertophy.