Cloud providers have recently introduced burstable instances-virtual machines whose CPU capacity is rate limited by token-bucket mechanisms. A user of a burstable instance is able to burst to a much higher resource capacity ("peak rate") than the instance's long-Term average capacity ("sustained rate"), provided the bursts are short and infrequent. A burstable instance tends to be much cheaper than a conventional instance that is always provisioned for the peak rate. Consequently, cloud providers advertise burstable instances as cost-effective options for customers with intermittent needs and small (e.g., single VM) clusters. By contrast, this paper presents two novel usage scenarios for burstable instances in larger clusters with sustained usage. We demonstrate (i) how burstable instances can be utilized alongside conventional instances to handle the transient queueing arising from variability in traffic, and (ii) how burstable instances can mask the VM startup/warmup time when autoscaling to handle flash crowds. We implement our ideas in a system called BurScale and use it to demonstrate cost-effective autoscaling for two important workloads: (i) a stateless web server cluster, and (ii) a stateful Memcached caching cluster. Results from our prototype system show that via its careful combination of burstable and regular instances, BurScale can ensure similar application performance as traditional autoscaling systems that use all regular instances while reducing cost by up to 50%.